Porous silicon samples were prepared by anodizing p-doped Si(100) substrates in both aqueous (HF/H2O, HF/CH3OH, HF/CH3OH/H2O, HF/C3H5OH/H2O) and nonaqueous (MeCN/HF) media. The time-dependent porous silicon photoluminescence (PL) was monitored during the etch (in situ) and after removal from the etch solution (ex situ). Correlation of the ex situ and in situ PL indicates that the composition of the etchant solution plays an extremely important role in the onset, time-dependent intensity, and Lifetime of the emission, both in and out of solution. The effect of etchant solution additives (ethylene glycol, CH3OH, C2H5OH, NaF, HCl, and NaCl) on the porous silicon PL both during and following the etching cycle, was also determined. The distinct and different correlations found between aqueous and nonaqueous etchants provide insights into the mechanism of FL. These results, when considered in the context of quantum chemical modeling, strongly suggest surface-bound silicon oxyhydride moieties as the source of the porous silicon PL.