Aerosols created by electronic cigarettes are suspensions of liquid droplets in a gas phase. All of the volatile or semi-volatile compounds in the system will partition between the two phases. Among these compounds are the "e-liquid" constituents plus the degradation products such as formaldehyde produced during "vaping". This partitioning affects deposition in the respiratory tract and optimal analytical method design. Theory can be used to predict the particle-gas distribution of each compound as a function of the mass concentration of the aerosol droplets (mu g/m(3)), the composition of the droplets, temperature, and the vapor pressure of the compound. As an example, even at the highest total particulate matter (TPM, mu g/m(3)) levels for e-cigarette aerosols, formaldehyde as CH2O will be mostly in the gas phase; two important adducts of formaldehyde will be mostly in the aerosol droplets even at the lowest TPM levels.