The onset of chaotic ion dynamics in electrostatic waves is investigated in a linear magnetized plasma. The existence of a threshold for heating in agreement with the predictions of single particle Hamiltonian theories and the fast time scale for velocity space diffusion are observed in the experiment. Measurements of test-particle dynamics indicate an exponential separation of initially close ion trajectories for amplitudes above the heating threshold. Ion orbits in the different wave-particle interaction regimes are inferred using an optical tagging diagnostic method, which provides the two point correlation function in phase space.