Recently, Mayol and Salvat have employed a semi-phenomenological appro
ximation along with the first Born approximation to calculate K-shell
ionization cross-sections of atoms. Their method requires optical osci
llator strengths (OOS) as input instead of the generalised oscillator
strengths (GOS). As the generation of the GOS for molecules is a diffi
cult task, in the present paper we have extended the semi-phenomenolog
ical approximation of Mayol and Salvat to investigate molecular ionisa
tion because of the availability of OOS for a large number of molecule
s. Cross-sections for the total ionisation and for the formation of th
e parent ions of H-2, N-2, O-2, NH3, H2O and CO2 by electron impact ar
e calculated in a modified first Born approximation for the incident e
nergy E varying from the threshold of ionisation to 1 keV. The contrib
ution of the exchange is included through the Ochkur approximation. Di
ssociative ionisation cross-sections of NH3, H2O and CO2 giving rise t
o NH2+ OH+ and O+ ions, respectively, have also been evaluated, The ca
lculated cross-sections are, in general, in good agreement with the ex
perimental data over a wide energy range (E greater than or equal to 5
0 eV) and the same holds for ratios of the partial to the total ionisa
tion cross-sections.