A proportional counter X-ray spectrometer, sensitive in the wavelength ranges 1–20 Å and 44–60 Å, was placed in orbit on the Orbiting Solar Observatory –4. The instrument began to acquire data on 1967 October 23. The spectrometer and its operation are briefly described. Its performance and the method of data analyses are also discussed. Preliminary studies have been made of the slowly varying component of solar X-radiation, the nature of the X-ray active regions in the solar corona and the characteristics of impulsive X-ray events.