Non-ionic surface-active agents having the formula: ##EQU1## in which R is alkyl, alkenyl, alkoxyalkyl and alkaryloxyalkyl, having 8 to 22 carbon atoms, X represents a CH.sub.2 OH or CH.sub.2 O--CH.sub.2 --CHOH--CH.sub.2 OH group, p and q each independently represent a number between 0 and 10 inclusive with the proviso that the sum of p and q is between 1 and 10 inclusive, and methods for preparing said surface-active agents comprising the step of polycondensing n molecules of glycerol epihalohydrin or allylglycidylether with a .alpha.-diol having the formula: ##EQU2## hydroxylating the resulting condensate and subjecting the resulting hydroxylated product to ethanolysis.