Abstract: A method for the preparation of polysaccharide derivatives comprises reacting at least a saccharide, such as glucose, maltose or glucose syrup, a "food grade" polycarboxylic acid, such as citric acid, or inorganic acid, such as phosphoric acid, which also acts as catalyst, and optionally a polyol, such as serbitol, under the influence of electromagnetic waves to give a reaction product at a temperature of 140.degree.-300 .degree. C. Preferably, the envisaged reaction is carried out in a commercially available microwave oven having a wave frequency in the range of 900-2500 MHz. The products obtained can be incorporated as low-calorie "bulking agents" in diet foods, such as dessert, cake and the like.