The vapor pressure of lithiunl over the system lithiunl oside -calcium oxide -silicon has been measured using the gas entrain~nent method a t temperatures between '370" C. and 1025' C. The experimental results are sumalarized in the forin 4 log P = 43.1 -( 6 . 6 9 / T ) X104, where P is in atmospheres and T i n A. 'The reaction, as usually written, does not describe the process conlpletely.