Differential barothermal analysis (DBA or HP-DTA) of the phase transformations in hypoeutectic Al-10 at.% Si and eutectic Al-12 at.% Si alloys was carried out at 100 MPa of argon pressure and at temperatures up to 700°C. Liquidus temperature of Al-10 at.% Si is reduced during melting and crystallization by 3-4°C as compared with the atmospheric pressure data. Increase in the eutectic transformation L ? (Al) ? Si temperature up to 580-581°C during heating was found for the Al-10 at.% Si and Al-12 at.% Si alloys. According to the barothermography data, a slight thermal effect at 553°C is detected for both alloys, which was identified as the decomposition of aluminum-based solid solution and precipitation of silicon particles. The improvement of silicon phase morphology for Al-10 at.% Si and Al-12 at.% Si alloys after hot isostatic pressing (HIP) at the pressure of 100 MPa and temperature of 550°C was found. No porosity in the alloy has been established after HIP treatment. After HIP thermal expansion, coefficients of the alloy are noticeably decreased by 25 (Al-10 at.% Si) and 50 % (Al-12 at.% Si).