The use of the felspar geothermometer (BARTH 1956, 1961, 1962) for the purpose of estimating the temperature of crystallization of two-felspar rocks depends on the determination of the albite content of both the alkali and plagioclase felspars. Each of these determinations is beset with difficulties, however. The alk.ali-felspar is frequently perthitic (often possibly a secondary condition), and, unless previously homogenized by heating at temperatures up to 900°C (TUTTLE and BOWEN 1958, pp.13 seq.), it is very difficult to separate from the whole rock sample for wet chemical or X-ray analysis. The plagioclase composition may sometimes be reliably determined by optical methods. However, secondary alteration, at least to fine-grained 'saussurite' minerals, is common, and, as this will have increased the albite content of the remaining plagioclase, a minimum figure only will be determined for the temperature of crystallization. The method described attempts to overcome these difficulties; only the results of mineral modal and wet chemical analyses are required.