We present femtosecond time-resolved measurements of the reflectivity of aluminum during the laser-induced solid-to-liquid phase transition over the spectral range 1.73.5 eV. Previous optical and electron diffraction studies have shown discrepancies on the order of picoseconds in the timescale of the solid- to-liquid phase change. As a result, it is not clear if the transition mechanism is thermal or non-thermal. Our experiments conclusively show that this transition is a thermal process mediated through the transfer of heat from the photoexcited electronic population to the lattice. Our findings agree with the results of the electron diffraction study and rule out the non-thermal mechanism proposed by the optical study.