Palladium spherical particles 0.23 mu m in diameter were synthesized by reducing palladium acetate with hydrazine in the presence of surfactant, with an aim of exhibiting both easy separation by filtration and easy dispersion for a catalyst. The particles in the suspension were sedimented slowly but not aggregated. The suspension showed voltammetric redox waves. The anodic wave was ascribed to the oxidation of Pd to Pd2+, whereas the cathodic one was to the reduction of the palladium acetate moiety to Pd. The current ratio of the anodic peak to the cathodic one 4:1, was close to the ratios by the partial chemical oxidation with permanganate and by the thermogravimetry, suggesting the composition of 80% palladium metal and 20% palladium acetate in the molar ratio. Heating the palladium particles at 300 degrees C yielded palladium metal. The decomposition proceeded to the first-order reaction with the activation energy of 40 kJ mol(-1). The particle catalyzed the reduction of methylene blue with hydrazine. The reaction rate was of the first-order with respect to methylene blue. The rate constant was proportional to the geometrical surface area of the palladium particle, suggesting a surface catalysis. (c) 2006 Elsevier Ltd. All rights reserved.