The solvent effect of ionic liquids on the lifetime of singlet molecular oxygen, O-2((1)Delta(g)), was investigated by means of time-resolved near-IR emission spectroscopy. O-2((1)Delta(g)) was generated by photosensitization of methylene blue in morpholinium and imidazolium ionic liquids, both comprising various alkyl chains of different lengths. The measured time profiles of O-2((1)Delta(g)) luminescence for the (1)Delta(g) -> 3 Sigma(-)(g) transition were represented by a single-exponential decay function. The phosphorescence lifetime was found to be correlated with the alkyl chain length of the morpholinium ionic liquids. This observation was interpreted by considering efficient quenching of O-2((1)Delta(g)) through energy transfer to the high-frequency C-H stretching modes of the N-alkyl chain in the morpholinium cation. Interestingly, we found that O-2((1)Delta(g)) quenching by the C-H stretching modes of the ring of the cations was remarkably depressed. The kinetics of the O-2((1)Delta(g)) emission decay process was discussed on the basis of the heterogeneous structure of ionic liquids consisting of polar and nonpolar domains