In this article we show the significant tunability of radiation chemistry in supercritical ethane and to a lesser extent in near critical CO₂. The information was obtained by studies of muonium (Mu = μ e^–), which is formed by the thermalization of positive muons in different materials. The studies of the proportions of three fractions of muon polarization, P_Mu, diamagnetic P_D and lost fraction, P_L provided the information on radiolysis processes involved in muon thermalization. Our studies include three different supercritical fluids, water, ethane and carbon dioxide. A combination of mobile electrons and other radiolysis products such as ^•C₂H₅ contribute to interesting behavior at densities ∼40% above the critical point in ethane. In carbon dioxide, an increase in electron mobility contributes to the lost fraction. The hydrated electron in water is responsible for the lost fraction and decreases the muonium fraction.