The preparation and application of a humic acids modified gold electrode (HA-CME) have been described. Derivatization of HAs as beta-thioesters caused their partial fragmentation and, thus, a consequent separation by size exclusion chromatography (SEC) was necessary. The CME prepared with functionalized HAs having an average MW of 52000 demonstrated to be effective for trace As(III) determination (LOQ = 0.3 microg L(-1)). This CME has been characterized both by electrochemical techniques and atomic force microscopy (AFM). The HA-CME provided reliable measurements in natural waters at different salinity with no need of desalting the sample. Total inorganic arsenic could be determined after reduction of As(V) with SO2. Under these conditions, organic arsenic species were not mineralized and did not interfere.