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This work describes the development of an electrochemical sensor that was used in the determination of ethinyl estradiol (EE) in pharmaceutical formulations, river water, and milk using the square wave voltammetry technique. Studies were carried out using different carbonaceous materials (multiwalled carbon nanotubes, reduced graphene oxide Reduced graphene oxide, graphite) and different metallic phthalocyanines (cobalt, iron and manganese). Based on these studies it was possible to obtain the best system for the construction of the sensor. The device was obtained by the chemical modification of a glassy carbon electrode (GCE) with multiwalled carbon nanotubes (MWCNTs) and cobalt phthalocyanine (CoPc). The materials were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Under conditions previously optimized for the proposed method, an analytical curve was constructed, presenting linearity in a range of 2.50–90.0 μmol L⁻¹ (R = 0.990), with detection limit of 2.20 μmol L⁻¹ and quantification of 2.50 μmol L⁻¹. The validation of the methodology for the determination of EE using GCE-MWCNTs-CoPc was performed, being accurate, precise, stable and sensitive. The recovery of ethinyl estradiol in the sample of pharmaceutical formulation was 103.93%, in the samples of river water ranged from 92.75% to 96.47%, and in the milk sample was from 88.00% to 96.20%. Thus, the proposed method presented a viable alternative for the determination of ethinyl estradiol in the quality control of pharmaceutical and food formulations as well as in environmental control. Full article