Secondary organic aerosol (SOA) formation in the atmosphere is currently often modeled using a multiple lumped "two-product" (<I>N</I>·2p) approach. The <I>N</I>·2p approach neglects: 1) variation of activity coefficient (ζ<sub><I>i</I></sub>) values and mean molecular weight <span style="border-top: 1px solid #000; color: #000;">MW</span> in the particulate matter (PM) phase; 2) water uptake into the PM; and 3) the possibility of phase separation in the PM. This study considers these effects by adopting an (<I>N</I>·2p)<sup>ζ<sub>p</sub><span style="border-top: 1px solid #000; color: #000;">MW</span>,ζ</sup> approach (θ is a phase index). Specific chemical structures are assigned to 25 lumped SOA compounds and to 15 representative primary organic aerosol (POA) compounds to allow calculation of ζ<sub><I>i</I></sub> and <span style="border-top: 1px solid #000; color: #000;">MW</span> values. The SOA structure assignments are based on chamber-derived 2p gas/particle partition coefficient values coupled with known effects of structure on vapor pressure <I>p</I><sub>L,<I>i</I></sub><sup>o</sup> (atm). To facilitate adoption of the (<I>N</I>·2p)<sup>ζ<sub><I>p</I></sub><span style="border-top: 1px solid #000; color: #000;">MW</span>,θ</sup> approach in large-scale models, this study also develops CP-Wilson.1 (Chang-Pankow-Wilson.1), a group-contribution ζ<sub><I>i</I></sub>-prediction method that is more computationally economical than the UNIFAC model of Fredenslund et al. (1975). Group parameter values required by CP-Wilson.1 are obtained by fitting ζ<sub><I>i</I></sub> values to predictions from UNIFAC. The (<I>N</I>·2p)<sup>ζ<sub><I>p</I></sub><span style="border-top: 1px solid #000; color: #000;">MW</span>,θ</sup> approach is applied (using CP-Wilson.1) to several real α-pinene/O<sub>3</sub> chamber cases for high reacted hydrocarbon levels (ΔHC≈400 to 1000 μg m<sup>−3</sup>) when relative humidity (RH) ≈50%. Good agreement between the chamber and predicted results is obtained using both the (<I>N</I>·2p)<sup>ζ<sub><I>p</I></sub><span style="border-top: 1px solid #000; color: #000;">MW</span>,θ</sup> and <I>N</I>·2p approaches, indicating relatively small water effects under these conditions. However, for a hypothetical α-pinene/O<sub>3</sub> case at ΔHC=30 μg m<sup>−3</sup> and RH=50%, the (<I>N</I>·2p)<sup>ζ<sub><I>p</I></sub><span style="border-top: 1px solid #000; color: #000;">MW</span>,θ</sup> approach predicts that water uptake will lead to an organic PM level that is more double that predicted by the </I>N</I>·2p approach. Adoption of the (<I>N</I>·2p)<sup>ζ<sub><I>p</I></sub><span style="border-top: 1px solid #000; color: #000;">MW</span>,θ</sup> approach using reasonable lumped structures for SOA and POA compounds is recommended for ambient PM modeling.