During the fourth Fire Lab at Missoula Experiment (FLAME-4, October–November 2012) a large variety of regionally and globally significant biomass fuels was burned at the US Forest Service Fire Sciences Laboratory in Missoula, Montana. The particle emissions were characterized by an extensive suite of instrumentation that measured aerosol chemistry, size distribution, optical properties, and cloud-nucleating properties. The trace gas measurements included high-resolution mass spectrometry, one- and two-dimensional gas chromatography, and open-path Fourier transform infrared (OP-FTIR) spectroscopy. This paper summarizes the overall experimental design for FLAME-4 – including the fuel properties, the nature of the burn simulations, and the instrumentation employed – and then focuses on the OP-FTIR results. The OP-FTIR was used to measure the initial emissions of 20 trace gases: CO₂, CO, CH₄, C₂H₂, C₂H₄, C₃H₆, HCHO, HCOOH, CH₃OH, CH₃COOH, glycolaldehyde, furan, H₂O, NO, NO₂, HONO, NH₃, HCN, HCl, and SO₂. These species include most of the major trace gases emitted by biomass burning, and for several of these compounds, this is the first time their emissions are reported for important fuel types. The main fire types included African grasses, Asian rice straw, cooking fires (open (three-stone), rocket, and gasifier stoves), Indonesian and extratropical peat, temperate and boreal coniferous canopy fuels, US crop residue, shredded tires, and trash. Comparisons of the OP-FTIR emission factors (EFs) and emission ratios (ERs) to field measurements of biomass burning verify that the large body of FLAME-4 results can be used to enhance the understanding of global biomass burning and its representation in atmospheric chemistry models. <br><br> Crop residue fires are widespread globally and account for the most burned area in the US, but their emissions were previously poorly characterized. Extensive results are presented for burning rice and wheat straw: two major global crop residues. Burning alfalfa produced the highest average NH₃ EF observed in the study (6.63 ± 2.47 g kg⁻¹), while sugar cane fires produced the highest EF for glycolaldehyde (6.92 g kg⁻¹) and other reactive oxygenated organic gases such as HCHO, HCOOH, and CH₃COOH. Due to the high sulfur and nitrogen content of tires, they produced the highest average SO₂ emissions (26.2 ± 2.2 g kg⁻¹) and high NO_x and HONO emissions. High variability was observed for peat fire emissions, but they were consistently characterized by large EFs for NH₃ (1.82 ± 0.60 g kg⁻¹) and CH₄ (10.8 ± 5.6 g kg⁻¹). The variability observed in peat fire emissions, the fact that only one peat fire had previously been subject to detailed emissions characterization, and the abundant emissions from tropical peatlands all impart high value to our detailed measurements of the emissions from burning three Indonesian peat samples. This study also provides the first EFs for HONO and NO₂ for Indonesian peat fires. Open cooking fire emissions of HONO and HCN are reported for the first time, and the first emissions data for HCN, NO, NO₂, HONO, glycolaldehyde, furan, and SO₂ are reported for "rocket" stoves: a common type of improved cookstove. The HCN / CO emission ratios for cooking fires (1.72 × 10⁻³ ± 4.08 × 10^&minus;4) and peat fires (1.45 × 10⁻² ± 5.47 × 10⁻³) are well below and above the typical values for other types of biomass burning, respectively. This would affect the use of HCN / CO observations for source apportionment in some regions. Biomass burning EFs for HCl are rare and are reported for the first time for burning African savanna grasses. High emissions of HCl were also produced by burning many crop residues and two grasses from coastal ecosystems. HCl could be the main chlorine-containing gas in very fresh smoke, but rapid partitioning to aerosol followed by slower outgassing probably occurs.