A new laboratory atomic sulfur beam source has been developed which has allowed the measurement of relative oscillator strengths for three prominent dipole-allowed SI transitions: 3p4 3P → 4s 3S0 (1814 ¿), 3p4 3P → 4s' 3D0 (1479 ¿), and 3p4 3P → 3d 3D0 (1429 ¿). The method of forward electron scattering was used. The oscillator strength ratios are found to be: f1479/f1814=1.12¿0.06 and f1429/f1814 =1.69¿0.08. If f1814 is normalized to the National Bureau of Standards reference value of 0.12, then: f 1479=0.134, f1479=0.202 (both ¿30%). These results are significant for the understanding of SI emissions observed from Io and the Io torus as well as from certain comets. ¿ American Geophysical Union 1990