The available evidence suggests that most of the nicotine in mainstream tobacco smoke is in the smoke particle matter (PM) phase. Nicotine can exist in protonated and free base forms in the smoke PM, and alpha(fb) is the fraction of the PM nicotine that is in the free base form. Because only the free base form can volatilize from the smoke PM phase to the gas phase of an inhaled aerosol and because gaseous nicotine deposits rapidly in the respiratory tract (RT), the magnitude and rate of nicotine deposition in the RT will depend on alpha(fb). The types of values that alpha(fb) can assume in the PM of cigarette smoke aerosols have not been well-known. The conventional view has been that mainstream cigarette smoke PM contains relatively little free base nicotine so that the cigarette smoker must absorb nicotine mostly from deposited particles. A prior study concluded that because cigarette smoke is at "pH 5.3", there is very little free base nicotine in such smoke. A 1994 internal tobacco company document discusses the view that "smoke pH" values for cigarette smoke are "approximately 6.0". This work uses volatility-based measurements to provide determinations of equilibrium nicotine alpha(fb) values for mainstream smoke PM from selected cigarettes. The effective pH (i.e., pH(eff)) of the smoke PM from selected brands of commercial cigarettes was found to span a range of 6.0-7.8 (nicotine alpha(fb) = 0.01-0.36), with all pH(eff) values much larger than 5.3 and most larger than 6.0.