A new red-region fluorogenic substrate, tetra-substituted amino aluminium phthalocyanine, was developed for the selective determination of H₂O₂ based on the catalytic effect of mimetic peroxidases, viz., hemin or iron tetrasulfonatophthalocyanine (FeTSPc). Under the optimum conditions, the linearity of the calibration graph for the determination of H₂O₂ with hemin (or FeTSPc) as the catalyst was in the range from 0.0 to 3.0 × 10⁻⁷ mol L⁻¹ (or from 0.0 to 2.0 × 10⁻⁶ mol L⁻¹). The detection limits were 3.7 × 10⁻⁹ and 4.9 × 10⁻⁹ mol L⁻¹ H₂O₂, respectively. The relative standard deviation (n = 7) was within 1.5% in the middle of the linear range. The peroxidase activity of the mimetic enzymes hemin and FeTSPc, the effects of some experimental conditions and the influence of foreign substances were investigated. With this substrate, 0.0–7.5 × 10⁻⁸ mol L⁻¹ hemin and 0.0–2.0 × 10⁻⁶ mol L⁻¹ FeTSPc can be determined with an accuracy and precision of about 1.3%. The potential application of the reagent was tested by the determination of H₂O₂ in rainwater.