The use of Cs to discriminate between igneous rocks derived from an igneous protolith (I-type) and those derived from a metasedimentary protolith (5-type) is evaluated for selected eastern Australian granitic and felsic volcanic rocks. For this investigation a ‘flame emission’ technique has been developed and tested for the rapid determination of Cs in rock samples. In the Berridale Batholith, where the I-type, S-type subdivision was first recognized, the Cs content of the granites clearly distinguished the two granite types. In the New England Batholith the leucocratic S-types have higher Cs abundances, in most cases, than the similar I-type granites, but the Cs values of the more mafic S-type granites overlap those of the I-type. Problems in assigning origins for volcanics are complicated by alteration, but Cs may be useful in distinguishing reworked tuffs from volcanic rocks in volcanic sequences. Caesium may also find application in distinguishing volcanics and sediments in Precambrian terrains.