Synopsis The number and type of phases in atmospheric organic particles consisting of internal mixtures of primary organic aerosol (POA) materials and secondary aerosol (SOA) materials is important to predict their effect on air quality and human health, as well as climate. Investigating the POA + SOA particles, we found that the number of phases depends on the oxygen-to-carbon ratio (O/C), with two-phase and single-phase POA + SOA particles formed for POA with low and high O/C’s, respectively.