Galacto-oligosaccharides (GOS) were synthesised from 50% (w/w) lactose with Aspergillus oryzae β-galactosidase. Reacted medium without nutrient supplementation (raw GOS) was fermented with Kluyveromyces marxianus cells obtaining GOS of 95% purity containing mostly tri- and tetrasaccharides with total recovery of GOS after 24h. The effect of the initial concentration of carbohydrate was assessed in the range of 10-50% (w/w), such purity being obtained even at 50% (w/w) (undiluted raw GOS) though being strongly influenced by the dry cell to total carbohydrate mass ratio (RCC). The process of purification by selective fermentation of raw GOS at 20% (w/w) was optimised in terms of the specific productivity of unwanted carbohydrate removal, considering temperature, pH and RCC as operational variables. According to the experimental design, maximum specific productivity predicted was 0.16gg-1h-1, obtained at the optimal operating conditions of 40°C, pH 3.5 and RCC 1.55gg-1.