Five igneous rock specimens from the Peninsular Ranges batholith of southern California have been analyzed for mineral content in a comparative study by X-ray diffraction (reference intensity technique) and optical polarizing microscopy. The X-ray method utilizes aerosol suspension of pulverized rock samples, followed by collection and analysis on glass fiber filters by powder X-ray diffractometry. Under some conditions, a standard well mount may be used for sample preparation, eliminating the need for sample transparency and matrix corrections. Optical analysis consists of standard identification procedures using polarized light followed by point counting. Excellent agreement between the two methods is observed for quantities of the major components for all rocks except an olivine gabbronorite. Discrepancies in the gabbronorite analyses are primarily due to sample inhomogeneities as well as the use of calculated reference intensity constants for the several solid solution minerals occurring in this sample. Where both experimental reference intensity constants and sample measurements are obtained using the aerosol suspension technique for sample preparation, relatively high accuracy and relatively rapid data collection may be achieved with the X-ray method.