Preparation:  The second half of 1959 was a busy year for Skylark, with nine being launched from July to December. SL12 was the 18th Skylark to be launched, and the 6th in 1959. The launch took place from Woomera.

Flight: An apogee of 132 km (82 miles) was successfully achieved.

Recovery:  Parachute recovery had not been introduced, and no information about the impact point has been found.  

Results: (i) The Birmingham University mass spectrometer measuring head was released on a long cable to avoid contamination. Unfortunately the experiment failed, but was repeated successfully on SL15 a week later. (ii) UCL's new solar physics Lyman-alpha detector was partially successful, and would be followed by many such experiments on Skylark and also British scientific satellites. (iii) The UCL rugged X-ray detector cameras survived impact and were recovered from 16 feet (4.9 m) under the Woomera desert and produced useable data! (iv) Nothing is known of the RAE technology tests.