The production of oligosaccharides using Penicillium citrium cells at high sugar concentrations was investigated at 50 C and pH 5.0. Both 1-kestose and neokestose were produced from sucrose, while both nystose and tetrasaccharide were produced from 1-kestose. However, no reaction product was obtained from neofructo-oligosaccharides such as neokestose. Based on these experimental results, a hypothetical reaction route was proposed to illustrate how neofructo-oligosaccharides are formed from 1-kestose.