We study the implications of generalized CP transformations acting on themass matrices of charged leptons in a model-independent way. Generalized$e-\mu$, $e-\tau$ and $\mu-\tau$ symmetries are considered in detail. In allcases the physical parameters of the lepton mixing matrix, three mixing anglesand three CP phases can be expressed in terms of a restricted set ofindependent "theory" parameters that characterize a given choice of CPtransformation. This leads to implications for neutrino oscillations as well asneutrinoless double beta decay experiments.