A derivative spectrophotometric method based on Kalman filter is described for the simultaneous determination of Ni, Cu and Zn with 4-(2-pyridylazo) resorcinol (PAR). The method offers the advantages of both Kalman filter and derivative spectrophotometry. First derivative molar absorptivities of Ni, Cu and Zn are 3.8×10~5, 1.6×10~5 and 6.2×10~5 L·mol~(-1)·cm~(-1), which are 5.8, 4.2 and 6.4 times of those in conventional spectrophotometry respectively. The result of the determination of the synthetic sample...