This paper reports on monometallic Pt nanoparticle (Pt-NP) and bimetallic SnPt nanoparticle (SnPt-NP) catalysts with different SnxPty alloy structures. The catalysts were fabricated using a polyalcohol reduction process, and the catalytic activity of each alloy phase for the hydrogenation of acetic acid to ethanol was investigated. High-resolution transmission electron microscopy (HR-TEM) results confirmed that SnPt-NP catalysts with different Sn/Pt atomic ratios can be successfully synthesized by controlling the Sn/Pt atomic ratios of each metal precursor (platinum(II) acetylacetonate and tin(II) acetate) in the starting mixture during a polyalcohol reduction process. Analyses by inductively coupled plasma spectroscopy and X-ray diffraction (XRD) indicated the formation of uniform Sn1Pt3 and Sn1Pt1 alloy structures in the SnPt at Sn/Pt atomic ratios of 0.32 and 1.09, respectively. Compared with the monometallic Pt metal phase in the Pt-NP catalysts, the Sn1Pt3 alloy phase markedly accelerated the hydrogenation of acetic acid. However, hydrogenation of acetic acid was not observed over the SnPt-NP catalysts at Sn/Pt=1.09, suggesting that the Sn1Pt1 alloy phase is inactive for the hydrogenation of carboxylic acids to corresponding alcohols. Therefore, we conclude that the Sn1Pt3 alloy phase is the most effective bimetallic SnPt alloy phase for catalyzing the hydrogenation of carboxylic acids.