The stability and oxidation pathways of arsenobetaine (AB), arsenocholine (AC), trimethylarsine oxide (TMAO) and tetramethylarsonium iodide (TMI) were studied in the presence of concentrated nitric acid (2.0 ml) and 30% hydrogen peroxide (0.20 ml) in a closed pressurized microwave digestion system by changing the microwave power (300–600 W). AB, TMAO and TMI were stable and remained intact up to 160 °C when digested at 400 W of microwave power for 15 min. With digestion at 300 W (145 °C) for 15 min AC was unstable and partially decomposed to TMAO (52 ± 4.8%). On increasing the microwave power to 400 W (160 °C) AC was readily converted to TMAO (97 ± 2.8%). With digestion at 500 W for 15 min (190 °C) AB was unstable and partially decomposed to TMAO (37 ± 3.3%). With digestion at 600 W for 15 min (207 °C) AB was completely transformed to TMAO (99 ± 5.0%). AC was not converted to AB during chemical oxidation. The separation of cationic arsenic compounds in the microwave digests was carried out on a reversed-phase HPLC column using HGAAS as an arsenic-specific detector. The proposed microwave-assisted digestion approach is promising and was applied to the dissolution and determination of total cationic arsenic compounds in environmental samples.