The identification of water-soluble arsenic species in a kidney of the Tridacna clam by electrospray quadrupole/time-of-flight mass spectrometry (ES Q-TOF MS) was investigated. The species were isolated by three-dimensional LC (size exclusion-anion exchange-cation exchange); the elution of arsenic was monitored by ICPMS. The average accuracy and precision of the molecular mass measurements, studied for a number of organoarsenic standards, were 22 (negative bias) and 15 ppm, respectively. Structural information was obtained in the tandem Q-TOF mode. A total of 15 organoarsenic species were identified, and 13 of these possessed the dimethylarsinoyl group (8 ribofuranosides, 4 acyclic compounds, and 1 dihydroxyfuran). Four of these species were previously unreported. Arsenobetaine and dimethylarsinic acid were also detected. The major species (accounting for up to 50% water-soluble arsenic) was 5-dimethylarsinoyl-2,3,4-trihydroxypentanoic acid. The metabolic interrelationships of these compounds and their significance are briefly discussed.