Electron impact ionization of formaldehyde has been studied using mass spectrometry. Cross-sections for the formation of ions are measured between 10 and 85 eV with a total cross-section of 4 x 10(-16) cm(2) at 50 eV. H2CO+, HCO+ and CO+ contribute to about 99% of the total cross-section at 85 eV. The branching ratio for the formation of these cations is 0.32 (H2CO+), 0.49 (HCO+) and 0.19 (CO+) between 45 eV and 85 eV. HCO+ is the most abundant above 32 eV. Three other minor ions, O+, CH2+ and CH+, are detected in the range 35-85 eV. (C) 2009 Elsevier B. V. All rights reserved.