Explorer 12 was a spin-stabilized, solar-cell-powered spacecraft instrumented to measure cosmic-ray particles, trapped particles, solar wind protons, and magnetospheric and interplanetary magnetic fields. It was the first of the S 3 series of spacecraft, which also included Explorers 14, 15, and 26. A 16-channel PFM/PM time-division multiplexed telemeter was used. The time required to sample the 16 channels (one frame period) was 0.324 s. Half of the channels were used to convey eight-level digital information, and the other channels were used for analog information. During ground processing of the telemetered data, the analog information was digitized with an accuracy of 1/100th of full scale. One analog channel was subcommutated in a 16-frame-long pattern and was used to telemeter spacecraft temperatures, power system voltages, currents, etc. A digital solar aspect sensor measured the spin period and phase, digitized to 0.041 s, and the angle between the spin axis and sun direction to about 3-deg intervals. The spacecraft functioned well until December 6, 1961, when it ceased transmitting data apparently as a result of failures in the power system. Good data were recorded for approximately 90% of the active lifetime of the spacecraft. The initial spin rate was 28.0 rpm, and the spin axis direction was right ascension 48 deg, declination -28 deg. The direction was nearly constant with time, and the spin rate slowly increased with time to 34.3 rpm. Apogee direction varied from about 1200 h to 0600 h local time.