Spectra from a thermal ionization mass spectrometer used for isotopic analysis of solids are recorded on a 10 in. pen recorder chart. An inexpensive semi-automatic measuring device has been made for quickly converting this spectrum into peak heights punched on computer cards. A computer programme calculates the isotopic analysis both as the percentage abundances of each isotope and as the ratios of all except one to a predetermined reference isotope. Statistical analysis based on Student's t distribution is provided, with further information of interest to the production of isotopes in electromagnetic separators.