Selected P-wave acoustic travel times through single crystal samples of olivine and garnet with different Mg/(Mg+Fe) ratios were measured at elevated temperatures (from 293 K to 363 K) using a newly developed GHz ultrasonic interferometer. These high precision data allow us to examine aspects of the elasticity-composition systematics of olivine (forsterite-fayalite) and garnet (pyrope-almandine) solid solution series. For every 10% decrease in the Mg/(Mg+Fe) content, the P-velocity of olivine along the b-axis decreases about 3% and ∂ ln C22/∂T decreases about 7%; for garnet ∂ ln[C44+(C11+C12)/2>/∂T decreases about 6%. If such velocity-composition systematics are typical and hold at the P-T regime of the mantle, only a 3--4% variation in the Mg/(Mg+Fe) content of olivine is required to explain the lateral seismic velocity variation in the upper portion of the mantle within one tectonic region (d lnVP~1%), and 7--8% variation in the Mg/(Mg+Fe) content to explain the 2% lateral seismic velocity variation of the upper mantle globally. ¿ American Geophysical Union 1996