The determination of ultratrace concentrations of molybdenum(VI) by adsorptive cathodic stripping voltammetry on a glassy carbon mercury film electrode is described. The method involves a controlled preconcentration of the element by interfacial accumulation as molybdenum‐8‐hydroxyquinoline (oxine) complex on the electrode followed by a cathodic stripping voltammetric measurement. The optimum analytical conditions for the measurement of molybdenum by this method include the use of 0.20 M acetate buffer at pH 5.25, 1 mM oxine, an accumulation potential of −0.40V (vs. Ag/AgCl) and a rotated electrode at 3000 rpm. Under these conditions, the linear concentration range and lowest detectable concentration obtained with a 5‐min accumulation were 0–300 μg L−1 (R2 = 0.997) and 0.5 μg L−1 [relative standard deviation (RSD) = 13.7%], respectively. The presence of most other metal ions do not interfere with the molybdenum determination, except for PbII and CdII which were successfully masked by addition of 3 μM EDTA, and TiIV and WVI which were masked by 1 μM tartaric acid. The interference of surface‐active substances, such as Triton X‐100, is overcome by UV irradiation of the sample. The use of the adsorptive stripping voltammetric technique, after decomposition by dry ashing and UV treatment, is successfully demonstrated for the determination of molybdenum in biological and environmental materials.