The energetic-particle satellite Vela-4 experiment(a) a pair of diffused consists of junction semiconductor detectors for electrons in the energyrange 30-600keV, and (b) a guardedfour-element, solar particle telescope for detectingprotons from 0.5-100MeV and alpha particles from 3.6-380 MeV, in 13 differentially-counted channels. Separation of protons from alpha particles is accomplished by an in-flight particle identifier. Two additional simplified telescopes are used to extend the anularcoverage of the primag telescope and a wide angle, highsensitivity scintillationdetector is employed to monitor low intensitysolar particle events. The design and operation of these instruments will be described. Copyright © 1968 by The Institute of Electrical and Electronics Engineers, Inc.