The production of methyl viologen radical cation (MV.+) in the radiolysis of aqueous solutions of 0.5 mM methyl viologen (MV2+) containing formate and tertiary butanol (t-BuOH) was studied using fast electron (integral yields) and 21 MeV He-4 ion beam pulses (track segment yields). The formate concentration was varied in the range of 10(-3)-2 M. It was confirmed that MV.+ is formed within microseconds and that it is stable in the measured time regions if the solution contains neither t-BuOH nor oxygen. Tertiary butanol radical formed through the scavenging of OH radical or H atom by t-BuOH reacts with MV.+, and this rate constant was determined as (8 +/- 2) x 10(8) M-1 s(-1). The effect of formate concentration on the yield of methyl viologen radical cation was measured in deaerated or N2O-saturated solutions. The radical yields in deaerated solutions agreed well with those measured by continuous ion beam radiolysis, which confirmed that this system can be used as a chemical dosimeter for ion beam pulse radiolysis, The yields of OH radicals were derived from the yields of G(MV.+) in the presence and the absence of t-BuOH. In the radiolysis with low linear energy transfer (LET) radiations, the calculated OH radical yields agree well with the values determined by other OH radical scavengers, formate and formic acid, except at high scavenging capacity. In He-4 ion beam radiolysis, the yields have a discrepancy in the measured scavenging capacity range. The sum of the contributions of e(aq)(-) and H atoms to form MV.+ was also determined. This value was lower than the yields estimated from other studies with He-4 ion beams. Yields are highly dependent on the nature of the solutes and combination of solutes because of intratrack reactions of the products.