Objective: To develop and validate simple, rapid and sensitive spectrophotometric method for the assay of four antimuscarinic antagonists, namely oxybutynin (OXB), solifenacin (SOL), tolterodine (TOL) and fesoterodine (FES) in bulk and pharmaceutical formulations.Methods: The proposed method is based on the reaction of the selected drugs with eriochrome cyanine R (ECR) in buffered aqueous solution at pH 1.0. The formed ion-pair complexes were extracted with dichloromethane and measured quantitatively with maximum absorption at 464 nm. All variables that affect on color intensity such as pH, buffer volume and concentration of ECR and extractive solvents were studied and optimized.Results: The calibration graphs were linear over the concentration range of 4–24, 4–32, 4–32 and 2–22 mg/ml for OXB, SOL, TOL and FES, respectively. The stoichiometry of the reaction was found to be 1:1 in all cases. Molar absorptivity values were found to be 2.043×104, 1.856×104, 1.798×104 and 2.856×104 l/mol/cm for OXB, SOL, TOL and FES, respectively. Excipients which used as an additive in commercial formulations did not interfere in the analysis.Conclusion: The developed method was successfully applied to determine OXB, SOL, TOL and FES in pharmaceutical preparations. The developed method can be used for quality control and routine analysis where time, cost effectiveness and high specificity of analytical technique are of great importance.