In order to characterize the organic matter present on the water (potable or wastewater) from a quick way and without chemical reagents (as the conventional method spends a lot of time and involves the use of permanent reagents) in this paper we obtained absorption spectrums of the phthalate potassium in function of oxygen concentration consumed by the phthalate in the conventional test (chemical oxygen demand (COD)). From the ratio of the absorbance and COD we obtained a calibration curve to measure the level of organic matter in the water. COD conventional tests were made and then compared with the COD found in the calibration curve obtained from Phthalate absorption. The tests were conducted with samples of wastewater from rice mill Roa of Espinal-Tolima and with treated water and wastewater from treatment plant at the University of Ibagué. Phthalate presents a characteristic absorption at 283 nm in the ultraviolet region where organic matter absorption is recognized. The comparison between the calibration curve and the conventional method presented absolute errors of 48, 24 and 4 mgO2/L, respectively. Although one of the errors gave high the results are satisfactory since the chemical test is only an indication with deviations of ± 30 mg O2/L.