Abstract: The development of methods for determining the pH of cigarette smoke since 1950's are reviewed. They fall into two groups. One determines the pH of whole mainstream cigarette smoke, and the other takes the pH of particulate matter only. The methods for whole cigarette smoke may involve either determining the pH of the aqueous solution of mainstream smoke, or measuring by direct insertion of a pH electrode into the smoke. The procedures for determining the pH of particulate matter are as follows. The particulate may be collected on a Cambridge pad or by electrostatic precipitation, then impregnated in water and determined the pH of the aqueous solution. In addition, the various methods for determining the amount of free nicotine are summarized. The relationships among smoke pH, free nicotine and impact of cigarette smoke are also discussed.