Particle formation was studied in a flow reactor with photolytic generation of sulfuric acid. Comparisons with previous results are a mixed bag with plenty of outliers. Addition of bases (dimethylamine and ammonia) led to large increases in particle numbers. Model simulations suggest that previous dimethylamine–sulfuric acid cluster thermodynamics are good, while a new set of the free energies of ammonia–sulfuric acid clusters was needed.