Nonionic cellulose ether-water systems have a characteristic feature of phase separation at certain temperatures, also termed the cloud point. The effect of various surfactants as additives on the phase behavior of nonionic cellulose ether, hydroxypropylmethyl cellulose (HPMC), has been studied. It was found that in the presence of ionic surfactants the cloud point (T(CP)) of HPMC decreased when small amounts of surfactant were added, and at higher concentration it increased. In the case of alkyltrimethylammonium bromides, surfactants with a longer alkyl chain (cetyltrimethylammonium bromide, CTAB, and tetradecyltrimethylammonium bromide, TTAB) influenced the T(CP) much more than that with a shorter alkyl chain (dodecyltrimethylammonium bromide, DTAB). Cetylpyridinium chloride (CPC) and cetylpyridinium bromide (CPB) were utilized to see the counterion effect on the T(CP) of polymer. Anionic surfactant sodium dodecyl sulfate (SDS) was found to be more effective as compared to its cationic counterpart with the same alkyl chain (DTAB), whereas nonionic surfactants showed no influence. From these observations it was concluded that the driving force was the interaction between the charged head groups of the surfactants and polar sites present in the nonionic HPMC. Gemini surfactants (alpha,omega-bis(hexa/tetradecyldimethylammonium)alkane dibromides) showed more influence on the T(CP) as compared to their conventional counterparts. The energetic parameters of clouding in HPMC in the presence of all surfactants used in the study were also calculated. Conventional surfactants had shown the contrast behavior of enthalpy and entropy changes at lower and higher concentration regions; i.e., Delta H(c)(0) and T Delta S(c)(0) were positive at low concentration of surfactants but negative at higher surfactant concentrations. In the presence of gemini surfactants, both the enthalpy and entropy changes were always negative.