Although ethanol possesses only two-thirds the energy density of gasoline, it has other properties that are beneficial to combustion in an internal combustion (IC) engine. These include a higher laminar flame speed and higher octane number relative to gasoline. The higher octane number of ethanol improves knock tolerance, and the faster flame speed provides potential benefits to the combustion process. Understanding these attributes will enable flexible fuel engines to benefit from some of the unique properties of ethanol. While data concerning the efficiency of ethanol fuels at varying compression ratios exist in the literature, there is a lack of fundamental combustion data to validate these results. As such, this work makes use of mass fraction burn (MFB) analysis to examine the differences between various blends of ethanol and gasoline, including the effect of the compression ratio relative to optimal combustion phasing, early and bulk burn rates, and combustion variability. Tests were carried out on a port-fuel-injected, electronically-controlled, modified single-cylinder cooperative fuels research engine operating at steady-state conditions and a stoichiometric air/fuel ratio. Combustion experiments were conducted at a constant engine load of 330 kPa net indicated mean effective pressure (NIMEP). To characterize the combustion process, in-cylinder pressure data were used to calculate MFB profiles. Combustion was examined as a function of the ethanol concentration, spark timing, and compression ratio. The experimental results indicated that higher ethanol blends increased the knock-limited compression ratio (KLCR). KLCR for the speed and load tested is 8 for gasoline with an octane rating of 91 research octane number (RON) as compared to 16 for an ethanol blend of 84% ethanol (E84). With an increased ethanol concentration in the fuel, the 0-10% MFB duration decreased. Of all of the fuel blends tested, pure gasoline had the longest burn duration in both 0-10% and 10-90% MFB intervals.