Absolute values of gas-phase absorption cross-sections for some atmospherically important molecules (HCl, H2O, H2S, NH3, H2O2, HNCO, CH4, CH3Cl, CH2Cl2, CHCl3, CHF2Cl, CHBr3, CH3CF2Cl and CH3CFCl2) have been measured at the H atom Lyman-alpha wavelength (121.567 nm) employing narrow band (Delta lambda = 0.0006 nm) laser radiation generated by resonant third-order sum-difference frequency conversion. The present values can be used to access the uncertainty associated with Lyman-alpha absorption cross-section values derived from available literature absorption spectra which usually have been measured with quite different and sometimes with quite low spectral resolutions.