Nanocrystalline FeS and FeSe compounds were prepared by solvothermal decomposition of a precursor complex [Fe(3)(μ(3)-O)(μ(2)-O(2)CCH(2)Cl)(6)(H(2)O)(3)]NO(3)·H(2)O in the presence of thiourea and sodium selenite, respectively. The as-obtained products were characterized by X-ray diffraction analysis (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and UV-vis spectroscopic techniques. Structural analyses revealed that the FeS and FeSe nanoparticles (NPs) are composed of needle-like and spherical particles, respectively. The FeS and FeSe NPs showed photocatalytic activity for the decomposition of rose bengal (RB) and methylene blue (MB) dyes under white light illumination. They also showed good catalytic activity toward oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H(2)O(2) and followed Michaelis-Menten kinetics. In addition, both FeS and FeSe NPs exhibited electrocatalytic activity toward reduction of hydrogen peroxide, which on immobilization on glassy carbon (GC) electrodes perform as amperometric sensors for detection of H(2)O(2). At pH 7.0, the FeS/GC showed a linear range for detection of H(2)O(2) from 5 to 140 μM, while for FeSe/GC the range was 5 to 100 μM.