Alkylene oxide polymerizations are performed in the presence of a double metal cyanide polymerization catalyst and certain magnesium, Group 3-Group 15 metal or lanthanide series metal compounds. The presence of the magnesium, Group 3-Group 15 metal or lanthanide series metal compound provides several benefits including more rapid catalyst activation, faster polymerization rates and the reduction in the amount of ultra high molecular weight polymers that are formed. The catalyst mixture is unexpectedly useful in making polyethers having low equivalent weights.