While biomass burning (BB) is the largest source of fine particles in the atmosphere, the influence of relative humidity (RH) and photochemistry on BB secondary organic aerosol (BB-SOA) formation and aging remains poorly constrained. These effects need to be addressed to better capture and comprehend the evolution of BB-SOA in the atmosphere. Cresol (C7H8O) is used as a BB proxy to investigate these effects. It is emitted directly from BB and has been identified as a significant SOA precursor from residential wood-burning emissions. The gas- and particle-phase signal intensities are investigated using online mass spectrometers. An increase in the SOA mass yield of 7% is observed when the RH rises from 0.5–20 to 70–87%. At elevated RH, a significant increase in the formation of nitrogen-containing compounds is observed due to particle-phase processes. This is linked to a net decrease in the SOA viscosity, enabling these compounds to be formed to a greater extent at elevated RH in the presence of nitrogen oxides. These results highlight the importance of the particle water content for the molecular formation and aging of BB-SOA compounds.