Oxygen deficiency has an immediate effect on both water and nutrient uptake, the yield of the whole plant under different soilless culture is affected. The oxygen level required for the respiration of the root system then becomes a limiting factor as much in soil culture as in soilless culture. The total area of soilless crops in SE Spain today is estimated to be about 5000ha. The objective of this experiment was to study the effect of potassium peroxide as an oxygen generator on vegetable crops growing in commercial substrates and to determinate the supply fraction for its use. Bioassay was used to determinate the fraction of potassium peroxide needed. Three greenhouse experiments were conducted in soilless culture with perlite and rockwool. Sweet pepper, melon and cucumber crop were supplied with potassium peroxide through fertigation to increase the oxygen content for one day per week. There were treatments: one with (T1) and the other without potassium peroxide (T0) in the nutrient solution. Fertigation parameters, yield and its quality were measured. Bioassay suggested that one gram per litre is the best fraction to use in soilless culture. Yield of sweet pepper was about 20% higher for T1 than T0 and 15% for melon; there was no significant difference in cucumber plants.