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Uranus and Neptune comma similar in size to the most abundant class of exoplanets comma have been undergoing temporal changes comma the causes of which we propose to investigate by measuring the informative spectral features of methane comma which has special diagnostic significance on these ice giants because it can condense in their cold atmospheres. Condensation makes methane.s local abundance a function of vertical and horizontal atmospheric motions and thus serves as an important constraint on models of such motions. Prior HST-STIS observations of Uranus in 2002 comma 2012 comma and 2015 have revealed that both polar regions have been depleted in methane. But a subsequent and continued dramatic brightening of its north polar region raises questions about the relative roles of aerosols and methane in producing this change comma an issue which new STIS observations can resolve. STIS observations of Neptune in 2003 suggest a methane depletion similar to that of Uranus comma but one sample does little to constrain whatever seasonal changes may be present. We thus propose 8 STIS orbits to obtain spatially resolved 524-1027 nm image cubes of both planets. These capture an important spectral region near 825 nm where methane and hydrogen absorptions are competitive and thus constrain the mixing ratio of methane relative to that of the dominant gas (hydrogen). They also capture a wide range of methane band strengths to help constrain the vertical distribution and scattering properties of aerosols comma especially aided by the additional constraint of center-to-limb variations. HST provides a combination of wavelength coverage comma spatial resolution comma and stability not available from any other facility.