The spin-orbit scattering (SOS) cross section σ_so of the 5(s, p) impurities is investigated in two different metal hosts. In the simple Mg host we observe a maximum at half filled p shell. This is the first experimental observation of an impurity p resonance and confirms theoretical predictions. For the transition metal host Cu the maximum in σ_so is shifted towards Te, the impurity with four p electrons. This is due to the hybridization of the Cu d states with the Te p level. The comparison between experimental and theoretical SOS cross sections represents a compact test of the quality of modern self-consistent electronic structure calculations.