Small amplitude oscillations of gas bubbles in sonicated liquids are quasi-reversible events, but the sudden collapse of widely expanded bubbles results in extensive vapor supersaturation. However, Hilgenfeldt et al. (Nature 1999, 398, 402) explain single-bubble sonoluminescence by assuming that collapsing bubbles revisit their equilibrium radii, R-o, filled with vapor-saturated gas ([gas]/[H2O] approximate to 31 at 300 K) before becoming impermeable and adiabatic en route to uniform greater than or equal to 25 kK temperatures. We find that the above assumption is physically untenable and seriously in error. In contrast, we calculate robust [gas]/[H2O] less than or equal to 0.12 ratios at R-o by using realistic alpha(H2O) less than or equal to 0.3 values for the mass accommodation coefficient of H2O molecules on liquid water at 300 K, and by taking into account the diffusive resistance developing within collapsing bubbles. Therefore, water vapor, rather than any particular gas, is the main component of collapsing bubbles. Its large heat capacity and atomization energies preclude reaching uniform peak temperatures exceeding 5 kK. We briefly analyze the consequences of this analysis and their relation to existing information.