Laser desorption/ionization (LDI) and/or laser ablation (LA) of selenium dioxide crystals or its mixtures with sodium peroxide were studied using a commercial matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer. It was found that LDI and LA of selenium (IV) dioxide not only ionizes SeO2, but also leads to the formation of several positively and negatively singly charged species: SeO (n = 0–2), Se, SeO (n = 0–4), Se2O (n = 3–7), Se3O (n = 4–9), Se4O (n = 8–10). A rather high yield of selenium species in the positive ion mode, Se (m = 1–8) and SemOH+ (m = 3–7), was obtained by using the MALDI approach while the species detected in the negative ion mode, SeO (n = 0–4), Se2O (n = 3–7), Se3O (n = 4–9), and Se4O (n = 9, 10), were the same as those observed during LDI/LA of selenium dioxide. The addition of sodium peroxide to selenium dioxide with the aim of enhancing its oxidation and thus increasing the production of SeO4 product resulted in extensive cationization of the species with sodium or potassium. The following positively and negatively charged species were identified: Se+, Se, Se2OH+, Se2ONa+, SeO (n = 0–3), and Se2O (n = 0, 1, 4). Also observed in mass spectra of such mixtures, various mixed sodium and/or potassium adducts with selenium oxide species, e.g. Se2O4K2Na−, were identified. In all, 26 totally new species, Se2O (n = 3–6), Se3O (n = 4–9), Se4O (n = 8–10), Se4O11H, Se4O12H, Se2O4Na−, Se2O5HNa−, Se2O5HNa, Se3O6K2Na−, Se3O6K2Na, Se2ONa+, and SemOH+ (m = 3–7), were described for the first time. Also, for the first time, the formation of selenium(IV) diperoxide, O-O-Se-O-O or O2SeO2, is described. The stoichiometries of the compounds generated were confirmed using isotopic pattern modeling. Copyright © 2006 John Wiley & Sons, Ltd.