The cosmic microwave background (CMB) anisotropies have been providing useful insights into neutrino physics in the past decade. This naturally arises the questions whether we can also test non-standard neutrino interactions using cosmological observations. After an introduction to cosmological perturbation theory, I present the Boltzmann hierarchy for neutrinos including self-interactions mediated by a massive scalar. Such interactions appear, for example, in majoron-like models of neutrino mass generation. I furthermore show the induced signal in the CMB angular power spectrum and compare our results to the approaches by others. Finally, I present constraints on the neutrino coupling, derived from a MCMC analysis of the Planck 2015 data.