Electronic nicotine delivery systems (ENDS) vaporize a solution (e-liquid) consisting primarily of the glycerin (GLY) and propylene glycol (PG) with nicotine added. The polyols typically make up 90% or more of the of the e-liquid solution. Nicotine in commercial products range from ~ 0.3 to 5 weight%. Nicotine in polyol solutions can be purchased on-line in concentrations up to ~10 weight % for use by consumers custom mixing e-liquids. There have been no published values of nicotine activity in these polyol mixtures. Equilibrium headspace was sampled from different mole fractions of nicotine mixed with the polyols. The mixtures were stored under nitrogen at room temperature in aluminum foil-lined gas bags for equilibrium. Nicotine partial vapor pressure was calculated from the nicotine headspace concentration. Both polyol systems had a positive deviation from ideal behavior, but the deviation from the GLY mixtures was much greater than that of the PG mixtures. For nicotine mole fractions of about 0.02 or less (< ~ 4 weight%), the GLY mixtures had nicotine activity coefficients of 11 while that for the PG mixtures was 1.5 indicating that the PG mixtures were much closer to an Ideal Solution. The Henry’s Law vaporization constant (initial slope for very dilute solutions expressed as Pa/mol fraction nicotine) was 51 to 53 in GLY and 5.3 in PG mixtures.