We have compared the frequency of high-redshift Hydrogen HI 21 cm absorption with that of associated molecular absorption in two quasars to place new ( 1 sigma ) upper limits on any variation in y \equiv g _ { p } \alpha ^ { 2 } ( where \alpha is the fine structure constant and g _ { p } is the proton g-factor ) of \Delta y / y < 5 \times 10 ^ { -6 } at redshifts z = 0.25 and z = 0.68 .