We propose a novel polarization demultiplexing technique for complex-modulated signals using the Stokes space polarization representation of the signal samples. Polarization demultiplexing of an endless time-varying SOP signal is achieved through the adaptive estimation of the inverse rotation matrix, without the calculation of a best fitting plane. Simulation results show the effectiveness of the proposed technique, with a high convergence speed, and a good response to time-varying SOP rotations. The performance of the method is assessed through EVM calculations for frequency rotations up to 15~$\mu$rad/symbol, showing a maximum penalty of 1 dB. Moreover, the technique can incorporate an additional stage for PDL compensation without penalties in terms of EVM results and algorithm complexity.