The role of a room temperature ionic liquid (RTIL, [pmim][Br]) on the size and conformational dynamics of a protein, horse heart cytochrome c (Cyt C) in its native, molten globule (MG-I and II), and unfolded states is studied using fluorescence correlation spectroscopy (FCS). For this purpose, the protein was covalently labeled by a fluorescent dye, Alexa Fluor 488. It is observed that the addition of the RTIL leads to an increase in the hydrodynamic radius (rH) of the protein, Cyt C in the native or MG-I state. In contrast, the addition of RTIL causes a decrease in the size (hydrodynamic radius, rH) of Cyt C unfolded by GdnHCl or MG-II state. The decrease in size indicates the formation of a relatively compact structure. We detected two types of conformational relaxation of the protein. The shorter relaxation time component (∼3-5.5 μs) corresponds to the protein folding or intrachain contact formation, while the relatively longer time component (∼63-122 μs) may be assigned to the motion of the protein side chains or concerted chain dynamics. The burst integrated fluorescence lifetime histograms indicate that the increase in size of the protein is accompanied by an increase in the contribution of the shorter component (∼0.3-0.4 ns) with a concomitant decrease of the contribution of the longer component (∼2.8-3.6 ns). An opposite trend is observed during the decrease in size of the protein. © 2012 American Chemical Society.