  QSO absorption spectra provide an extremely useful probe of possible
cosmological variation in various physical constants. Comparison of H I 21cm
absorption with corresponding molecular (rotational) absorption spectra allows
us to constrain variation in y=alpha^2*g_p where alpha is the fine structure
constant and g_p is the proton g-factor. We analyse spectra of two QSOs, PKS
1413+135 and TXS 0218+357, and derive values of dy/y at absorption redshifts of
z=0.2467 and 0.6847 by simultaneous fitting of the H I 21cm and molecular
lines. We find dy/y=(-0.20 +/- 0.44)*10^{-5} and dy/y=(-0.16 +/- 0.54)*10^{-5}
respectively, indicating an insignificantly smaller y in the past. We compare
our results with other recent constraints from the same two QSOs (Drinkwater et
al. 1998; Carilli et al. 2000) and with our recent optical constraints which
indicated a smaller alpha at higher redshifts.
