Abstract: The kinetics of the condensation reaction of o-toluidine and p-nitro-sophenol to produce o-methyl-indoaniline, and the quantitative determina-tion of the latter have been studied photometrically. Attempt to determine the product by first converting it to its leuco form with excess reducing agent (SnCl2, Vitamine C) has been proved unsatisfactory, because the extent of excess of the reducing agent will notably affect the analytical results. It has been found from photometric data that the o-methyl-indoaniline itself is very stable in a solution of about 82% H2S04. If enough sulphuric acid of proper strength has been used to dilute the sample of reaction mass so as to stop the reaction almost instantaneously and completely, then the quantity of indoaniline present will not change with time. Meanwhile, samples of pure o-methyl-indoaniline and p-nitrosophenol have been prepared, and their characteristic absorption curves in 82% H2S04 solutions have been determined. It is found that: o-methyl-indo-aniline has a max. absorp. at 574 mμ, and its molar extinction coeff. is ∑_(574)~In= 2.44 × 104 (l.-mol-1 -cm-1), while p-nitrosophenol does not interfere at this wave lenth. At 428 mμ, the molar extinction coeff. of o-methyl-indoaniline is ∑_(428)~In= 2.27 ×103; of p-nitrosophenol is ∑_(428)~N.P.= 5.4 × 103. There-fore, it is possible to analyze individually these two components in a mi x-ture by measuring the light absorption at 574 & 428 mμ with a photometer. Depending upon these, the composition of successive samples of reaction mass has been analyzed photometrically, and its change with time has been recorded. The calculated values of the rate constant k and the energy of activation E have been found to be quite consistent with the data found in the literature. The variation of k values with the concentration of reaction medium (H2S04) has been found also consistent with those data. Therefore it has been proved possible to apply photometric method in analysis of o-methyl-indoaniline (and p-nitrosophenol) in the reaction, mass or in control of the production process. In applying this method to study the progress of the condensation reaction under practical production condi-tions, it is discovered that the amount of o-methyl-indoaniline present in the reaction mass steadily in creases to a maximum, and then begins to decreases. These points may be of some significance in the practical production.