An on-site determination method for trace manganese has been developed using the manganese-catalyzed oxidation of Malachite Green (MG) with potassium periodate. Absorbance measurement of MG was carried out using a laboratory-made palm-top size colorimeter with a red-green-blue light-emitting diode. The reciprocal value (lit) of the reaction time at a fixed absorbance value for red light was chosen as a kinetic parameter for simple on-site analyses. The value of lit was proportional to the concentration of manganese in the range of 2 - 20 mu g L(-1). At a reaction temperature (7) of 20 degrees C or more, manganese was determined within a reaction time of 25 min. The calibration equation was approximated by 1/tau = (aT + b)[Mn] + (cT + d), where a to d were constants in a range of 10 - 40 degrees C. The two equations for 30 degrees C (for the laboratory-preparation of the calibration equation) and T give the value of lit at 30 degrees C as 1/tau = (30a + b)(1/tau - cT - d)/ (aT + b) + 30c + d. Without any temperature control, 1/tau(f) can be calculated by this equation and measurements of lit and T. The calculation introduced analytical errors of within 1 mu g L(-1). The proposed method was successfully applied to tap-, river- and lake-water samples.