Detonation wave theory was applied to the physical process of a vapor explosion. Initially, our experimental observations using hot water as the fuel and saturated refrigerant liquid as the coolant were analyzed with this technique. These tests are notable since peak explosion pressures were far below the critical pressure of the coolant. From the analysis, the volume fractions of the coolant vapor and the volume ratio of the two liquids prior to the explosion were estimated from the measured peak explosion pressures and associated explosion propagation velocities under the assumption that the process was steady and one-dimensional. Complete Hugoniot curves were constructed, and the detonation condition was initially determined under the assumption that flow velocity behind the shock was equal to the mixture sound speed. This assumption was checked with the tangency condition between the Rayleigh line and Hugoniot curve at the Chapman-Jouguet point, as well as the existence of a minimum in the entropy change across the shock wave. The point of minimum entropy showed good agreement with the graphical tangency point, but was slightly different than the sound speed criteria in pressure (<2%) with a larger difference in propagation speed (50%). This discrepancy between the three criteria becomes insignificant as the explosion pressure rises. This is demonstrated by examining a tin-water explosion experiment. This technique appears to be a useful tool to estimate initial conditions for subcritical vapor explosions.