We synthesized a compound, a tetraol, which is an atmospheric oxidation product in isoprene-derived secondary organic aerosols, and studied whether the tetraol is liquid or solid depending upon temperature and relative humidity, both in pure form and in mixtures with other compounds. Our results imply a liquid state of isoprene-derived aerosol particles in the lower troposphere at moderate humidity, but a solid state at colder upper tropospheric conditions, thus supporting modeling calculations.