Irosin ignimbrite (41 cal kBP) and its related deposits were the first widespread tephra recognized in the Philippines. This paper describes petrographic properties such as the particle composition and refractive index of the volcanic glass, in order to understand the nature of the eruption that emplaced these deposits. Green hornblende, cummingtonite, and biotite were observed in most of the samples analyzed, but green hornblende was not observed in some pumice clasts from the ignimbrite. However, it occurred in the matrix of the ignimbrite, co-ignimbrite ash, and plinian pumice samples. These results are probably due to small amounts of green hornblende phenocryst in the pumice fragments. The coexistence of cummingtonite and biotite in pumice indicates differences in crystallization conditions within the magma reservoir. A basal ash from the Malobago lava dome that preceded the ignimbrite eruption contains isolated crystals of orthopyroxene and irregular glass shards. The pumiceous deposit within the caldera shows slightly different petrographic properties (e.g. wider range of refractive index) from the other samples, which supports the interpretation of Irosin ignimbrite being reworked within the caldera.