Fourier transform infrared spectroscopy (FTIR) provides a unique technique to study membranes and proteins within their native cellular environment. FTIR was used here to study the effects of dimethyl sulfoxide on membranes and proteins in human pulmonary endothelial cells (HPMECs). Temperature-dependent changes in characteristic lipid and protein vibrational bands were identified to reveal the effects of on membrane phase behavior and protein stability. At concentrations equal to or below 10% (v/v), was found to decrease membrane conformational disorder. At higher concentrations (15% v/v), however, membrane conformational disorder was found to be similar to that of cells in the absence of . This effect was observed over a wide temperature range from down to . had no clear effects on cellular proteins during freezing. During heating, however, had a destabilizing effect on cellular proteins. In the absence of , protein denaturation started at an onset temperature of , whereas at 15% the onset temperature of protein denaturation decreased to . This implies that in the presence of the onset temperature of protein denaturation is lower than the normal growth temperature of the cells, which could explain the well documented toxic effect of at physiological temperatures. destabilizes cellular proteins during heating and decreases membrane conformational disorder over a wide temperature range.