This paper presents investigations on the dynamic behaviour of DMFCs. Experimental cell voltage responses to cell current step changes are investigated with transfer function analysis. The voltage response in most places shows significant overshooting behaviour (up to 80%) when applying cell current steps to the DMFC. Linear system analysis is applied to a set of dynamic models with different complexity, and a criterion for cell voltage overshooting is developed. In all models the dominating physico-chemical phenomenon is the transport of methanol through the membrane, causing cathodic overpotential overshooting. This is supported by experimental analysis. The proposed models and transfer functions are suitable for controller design.