The deposition of asphaltene during oil recovery has large economic and technical impacts on the production. The use of surfactant based asphaltene inhibitors is widespread but the effect at reservoir conditions is not well documented scientificly at elevated pressure. In the present paper the effect of two different commercial inhibitors and their effects on the onset pressure (asphaltene deposition envelope) are reported as a function of temperature using an optical laser light transmission technique and bottomhole oil samples. A high-pressure dosing procedure has been developed such that injection in the order of ppm's can be performed without disturbing these samples. Large concentrations of inhibitor (1000 ppm) are needed in order to observe a significant effect. The possible mechanisms of inhibition and the negative effect at low dose is discussed. Also, the detemination of the asphaltene precipitation onset pressure from the light transmission versus pressure signals is discussed in detail.