Calculation of the gas phase proton affinity of CH4, PA(CH4), involves characterisation of the CH5+ ion whose fluxional nature requires reevaluation of some assumptions routinely made for more normal molecules. Here we determine the PA(CH4; 0 K) using quantum diffusion Monte Carlo (QDMC) simulations on a previously developed CCSD(T)/aug-cc-pVTZ potential energy surface for CH5+. We obtain a value of 542.4 kJ/mol in reasonable agreement with the most recent measurements. The results obtained from standard thermochemical methods, and the limitations of these methods, are also discussed.