Capillary oscillations on modulated liquid jets have been investigated using laser illumination and electronic detection of the magnified jet shadow. The amplitudes of several wave harmonics of a growing spatial instability were measured as a function of distance from the orifice for a range of jet velocities and initial‐disturbance amplitudes. The experimentally determined growth rates at the fundamental frequency are compared with theories of capillary‐wave propagation. An empirically derived explanation of the suppression of satellite formation is given. Experimental evidence for infinite‐wavelength capillary oscillations is reported; a description of these oscillations in terms of the Rayleigh theory is presented.