Aluminum matrix composites reinforced with mixtures of diamond and SiC particles of equal size were produced by gas pressure assisted liquid metal infiltration. Replacing SiC gradually by diamond particles results in a steady increase of thermal conductivity from 220 W/mK to 580 W/mK. Electrical conductivity measurements indicate that the silicon content in the matrix decreases with increasing diamond volume fraction. Predictions of the differential effective medium scheme generalized for multiple types of inclusions agree well with experimental results.