During high power ICRF heating,the distribution function of the resonating ions becomes non-Maxwellian and an anisotropic taildevelops mainly in the perpendicular velocity direction. The evolution of the distribution functioncan be described by a quasi-linear diffusion operator in phase space. Owing to finite Larmor radius(FLR) effects, the diffusion coefficient in this operator varies with the gyro-radius normalized tothe wavelength. At certain ion energies the diffusion coefficient can become strongly reduced,effectively preventing resonating ions from reaching higher energies. Measurements are reported which show a significant difference in the distribution function between differentscenarios for heating of hydrogen ions in JET. Comparison with theoretical calculations shows thatthese differences can be explained by higher order FLR effects on velocity space diffusion duringICRF heating. The importance of these effects on the power deposition is also emphasized.