The effect of organic amendments on the biodegradation of n-alkanes and phenanthrene in diesel contaminated soil was evaluated. The organic amendments used were fermented and non fermented sweet milk whey and the mineralization was estimated by analysis of evolved 14CO from added 14C-hexadecane, 14C-octadecane and 14C-phenanthrene. The addition of 2100 mg dry weight (dw) sweet whey kg-1 soil dw at the start of the experiment increased the extent of mineralization after 184 days compared to a control, from 45 % to 61 % for n-hexadecane and from 35 % to 46 % for n-octadecane. The stimulation was characterized according to a three-half-order kinetic such as if a greater part of the substrates was mineralized according to first order kinetics. Similar stimulation was achieved from the fermented whey when 210 mg dw kg-1 soil dw was added to the soil every second week during the 184 days of incubation.