Isomerization of lactose to lactulose using sweet cheese whey ultrfiltrate with sodium hydroxide as catalyst was studied. The effects of parameters, such as the catalyst concentration, pH, reaction time and temperature, on the reaction rate and conversion of lactose to lactulose and by-product formation were investigated. Whey ultrafiltrate, a pH value of 11.15 and a temperature of 70degreesC were found to be suitable for this reaction.