If charged particles propagate through a magnetized plasma, they experience different scattering effects. Well-known effects that have been studied previously include spatial diffusion and pitch-angle scattering as well as stochastic acceleration. So-called gyrophase diffusion, which describes stochastic changes of the gyrophase due to interaction with magnetic turbulence, has been ignored in previous works. Therefore, we compute gyrophase diffusion coefficients by employing quasi-linear theory as well as a second-order approach. We then compute the velocity distribution function by solving the corresponding differential equation. These results are important for testing previous assumptions employed in particle diffusion theories. As an application, we compute velocity correlation functions and diffusion coefficients along and across the mean magnetic field.