In the present work, methyl viologen (1,1'-dimethyl-4,4'-bipyridinium dichloride) is used as a scavenger to estimate the radiolytic yields of water decomposition products from room temperature to 400 degreesC by pulse radiolysis method. {G(e(aq)(-)) + G(OH) + G(H)} has been studied using a 0.5 mM MV2+ solution in the presence of 10 mM NaCOOH up to 200 degreesC and in the presence of 0.2 M ethanol up to 400 degreesC. The results show that the {G(e(aq)(-)) + G(OH) + G(H)} increases with temperature up to 350 degreesC at 25 MPa, while it depends also on pressure in supercritical conditions. The G(eaq-) was estimated using MV2+ solutions in the presence of 0.2 M tert-butyl alcohol. The results agree well with the reported data up to around 300 degreesC at 25 MPa; however, in supercritical conditions a very significant density effect was observed. At a given temperature, G(e(aq)(-)) and {G(e(aq)(-)) + G(OH) + G(H)} decrease with increasing density while at a fixed density they decrease with increasing temperature.