Abstract Absolute Balmer line emission cross sections are determined. The Balmer emission is obtained by dissociative electron excitation in propane, propylene and n -butane. The optical excitation functions measured for these lines are investigated in the medium energy range of 50–700 eV and normalized by a He bench-mark procedure. The measured data are corrected by an apparatus collection efficiency factor F to compensate for the loss of optical signal due to non-thermal energies of H-excited molecular fragments. The obtained results are determined with an accuracy of ± 15%. The absolute Balmer line cross sections are analysed according to the Bethe theory to get some information on primary excited parent molecular states. The power-law dependence for Balmer series has been determined.