Abstract: A simple, accurate, specific and reliable colorimetric method was developed for the quantification of amoxycillin in different salt forms in different formulations. On treating amoxycillin with acidic formaldehyde, intra molecular aminolysis occurs. As a result, α-amino benzoyl group present in the side chain of Amoxycillin attacks the carbonyl group present in β-lactam ring nucleophilically, resulting in the formation of pyrazine derivative. The λmax was found to be 341 nm. This helps in the determination of amoxycillin without any interference from sulbactam. The method was found to be linear from 5.8 to 40.6 μg mL-1 with a correlation coefficient of 0.999.