The heterocyclic nitramine compounds, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), are two most important military-purpose high explosives. Differentiation of RDX and HMX with colorimetric methods of determination has not yet been made because of their similar chemical structures. In this study, a sensitive colorimetric method for the determination of RDX and HMX was proposed on the basis of differential kinetics in the hydrolysis of the two compounds (yielding nitrite as a product) followed by their colorimetric determination using 4-aminothiophenol (4-ATP) modified gold nanoparticles (AuNPs) and naphthylethylene diamine (NED) as coupling agent for azo-dye formation, abbreviated as "4-ATP AuNP+NED" colorimetric method. After alkaline hydrolysis in a 1 M Na2CO3 + 0.04 M NaOH mixture solution at room temperature, only RDX (but not HMX) was hydrolyzed to give a sufficient colorimetric response in neutralized solution, the molar absorptivity (epsilon) at 565 nm and the limit of detection (LOD) for RDX being (17.6 +/- 1.3) x 10(3) L mol(-1) cm(-1) and 0.55 mu g mL(-1), respectively. On the other hand, hot water bath (at 60 degrees C) hydrolysis enabled both nitramines, RDX and HMX, to give substantial colorimetric responses; i.e., epsilon and LOD for RDX were (32.8 +/- 0.5) X 10(3) L mol(-1) cm(-1) and 0.20 mu g mL(-1) and for HMX were (37.1 +/- 2.8) X 10(3) L mol(-1) cm(-1) and 0.24 mu g mL(-1), respectively. Unlike other AuNP-based nitrite sensors in the literature showing absorbance quenching within a relatively narrow concentration range, the developed sensor operated with an absorbance increase over a wide range of nitrite. Synthetic mixtures of (RDX + HMX) gave additive responses, and the proposed method was statistically validated against HPLC using nitramine mixtures.