The orientations Si-crystals take in aluminium, in an alloy with composition Al-1.3at%Si, were investigated by transmission electron microscopy. Hardness was measured for isothermal heat-treatments at 175 degrees C and 260 degrees C. Conditions analysed by TEM were 17 h at 175 degrees C and an additional 3 h at 260 degrees C, both containing a high density of small Si-crystals, the finest corresponding to 175 degrees C. Two main orientation relationships were found: (001)Al parallel to (001) Si, [100] Al parallel to [100]Si and (001) Al parallel to (110)Si, [1 (1) over bar(1) over bar ]Si parallel to [010]Al. The first accounted for approximately 60% of Si precipitates in condition 17 h_175 degrees C. Despite a high number density and well-aligned interfaces, the Si precipitates have negligible influence on hardness. Findings are consistent with Ge particles in Al-Ge alloys.