A glassy carbon electrode (GCE) coated with Langmuir–Blodgett (LB) film of p-allylcalix[4]arene is described for simultaneous determination of traces of thallium and cadmium in environmental water. The LB film electrode in a 0.1M KH 2 PO 4 solution shows a linear voltammetric response in the range of 5–250μgl −1 for thallium and 10–300μgl −1 for cadmium. The detection limits of thallium(I) and cadmium(II) are estimated to be 1.0μgl −1 (ca. 5.0×10 −9 M) and 2.2μgl −1 (ca. 2.0×10 −8 M), respectively. Above all, the indium does not respond to this LB film electrode.