We present experimental observations for the rotational state selection of formaldehyde. Data were acquired by using ail electrostatic hexapole to focus a supersonic beam of formaldehyde seeded in argon into the ionizer of a quadrupole mass spectrometer. Empirical focusing spectra agree with theoretical predictions when a rigorous quantum mechanical treatment is used to calculate the Stark effect for the rotational states of formaldehyde. These results have implications in the production of beams of cold molecules. Published by Elsevier B.V.