The following linear equation correlates the cloud point (CP) of water-soluble polyoxyethylated nonionic surfactants (NSs) with the average number p of oxyethylene units per molecule: (p - p(0))/CP = a + b(p - p(0)). Here p(0) is the smallest value of p that confers solubility in cold water: In a homologous series of NSs, it belongs to the surfactant with CP = 0degreesC. Plots of CP versus p for five representative homologous series of NSs consist of three segments: A steeply ascending, nearly straight line, a transition region that ranges from p = 15-22 to p = 20-28, and a nearly horizontal plateau that approaches asymptotically the CPs of polyethylene glycols with-molecular weights between 30,000 and 4400. These CPs range from 113 to 130degreesC, Most CPs for NSs were taken from the literature or measured on commercially available samples; eight CPs above 100 degreesC were measured on newly synthesized surfactants. Previously published linear equations correlating CP with p cover only NSs with p < 16 and CPs < 100degreesC: They apply only to the ascending segment of the CP versus p plots. Our equation covers the entire plots and applies to the full range of NSs, including extensively polyoxyethylated NSs with p greater than or equal to 100. It can be used for selecting specific NSs for high-temperature applications, The hydrophile-lipophile balance of the surfactant with p = p(0) oxyethylene units, namely, HLB0, is a novel quantitative measure of the hydrophobicity of the hydrocarbon moiety of the relevant homologous NS series. Its value reflects the size, composition, and structure of the hydrocarbon moiety. (C) 2003 Elsevier Science (USA). All rights reserved.