This work aims to develop a sensitive and easily applicable gold nanoparticles (AuNPs)-based colorimetric sensor for determining the propellant ammonium perchlorate (AP) in the presence of other explosives and interferents. The sensor relied on attaching methylene blue (MB), a phenothiazine dye with a planar structure, onto negatively charged AuNPs, and exploited the dyes electrostatic interactions with the large oxyanion, perchlorate. Ion-pair formation between MB@AuNP nanocomposite and AP led to nanoparticle aggregation, causing a red shift in the surface plasmon resonance (SPR) band of AuNPs. This new peak emerged at around 650 nm, giving rise to the development of a ratiometric sensor by taking the ratio of 650 nm absorbance to that at 520 run (pertaining to nanocomposite) and correlating this ratio to AP concentration. The limit of detection (LOD) and limit of quantification (LOQ) of the sensor for AP were 2.4 x 10(-5) and 8.3 x 10(-5) mol L-1, respectively. The recovery of AP from ternary mixtures containing TNT and HMX was accomplished by water extraction using solubility differences. The interferences of common soil ions such as Cl-, NO3-, NO2-, SO42-, CH3COO-, K+, Me2+ and Ca2+ were investigated, and the assay was statistically compared (using t- and F-tests) against a reference method. The proposed method could accurately determine perchlorate in a sparkler: sample.