Addition of carbon as methanol or ammonium carbonate to the aqueous analyte solutions in combination with increased plasma power input enhanced the inductively coupled plasma mass spectrometry (ICP-MS) signal intensities of arsenic and selenium. In the presence of the optimum 3% v/v methanol concentration the signal intensities achieved were about 4500-5000 counts s^-1 per ng ml^-1 of arsenic and about 700-1100 counts s^-1 per ng ml^-1 of selenium (⁸²Se), corresponding to enhancement factors of 3.5-4.5 compared with aqueous solution for the two elements. Differences in sensitivity (calculated on the basis of analyte atom) were observed between the individual arsenic species and between the selenium species in aqueous as well as in carbon-added solutions. The presence of 3% v/v methanol in the analyte solutions doubled the level of the background signal for arsenic and selenium, but its fluctuation (noise) was not increased. Therefore, the observed increase in analyte sensitivity led to a similar increase in signal-to-noise ratio. The addition of carbon as ammonium carbonate enhanced the arsenic signal by a similar factor but caused severe contamination of the ICP-MS instrument by carbon. In the 3% v/v methanol solutions of arsenic and selenium the signal intensity from antimony (internal standard) was enhanced by a factor of 1.5, which indicates that the enhancement effect of the arsenic and selenium signals by methanol is only to a limited extent caused by improved sample transport/nebulization efficiency. It is proposed that an increased population of carbon ions or carbon-containing ions in the plasma facilitates a more complete ionization of analytes lower in ionization energy than carbon itself. The enhanced detection power for arsenic was applied to arsenic speciation by high-performance liquid chromatography (HPLC)-ICP-MS, and made possible the detection of the arsenocholine ion (AsC) in extracts of shrimp at the 5-10 ng g^-1 concentration level. The limit of detection was improved by a factor of 3.4 after addition of methanol and was 4.7 ng g^-1 as the AsC ion.