Abstract: In a H3PO4 medium, the complex anion I3- was formed by the oxidation of I- with H2O2; and in the solution of aliphatic alcohol-polyoxyethylene ether sodium sulfate (AOES), an ion-association complex was formed by the reaction of I3- with methylene blue, having the maximum absorption wavelength at 530 nm. Range of linearity and value of apparent molar absorptivity found were 1.7×10-6-2.5×10-5mol·L-1 and 4.12×104L·mol-1·cm-1 respectively, and detection limit (3s/k) was 2.1×10-7mol·L-1. Based on these factors, a method of spectrophotometry for determination of H2O2 in atmospheric precipitation was proposed. Recovery of the method was tested by standard addition method, values of recovery found were in the range of 94.3%-105% with RSD′s (n=5) in the range of 1.4%-3.2%.