The factors that control the electrocatalytic activity for the oxygen reduction reaction (ORR) in fuel cells have been investigated systematically with the low cost carbon-supported Pd100-xCox (0 <= x <= 50) nanoalloys. The Pd100-xCox/C samples have been prepared by a solution-based reduction procedure and annealed at 350 and 500 degrees C. While the catalytic activity of the as-prepared samples decreases monotonously with nominal Co content due to poor alloying and instability, a volcano type dependence of the activity on Co content with the maximum at around a nominal Co content of 25 at% Co or an actual Co content of about 10 at% in the Pd lattice (degree of alloying) is found for the samples annealed at 350 and 500 degrees C due to the influence of both the degree of alloying and variations in crystallite size. Below 30 at% nominal Co content, the catalytic activity of the annealed samples increases with increasing nominal Co content due to both a decreasing crystallite size and increasing degree of alloying. For 30-50 at% nominal Co content, the catalytic activity decreases with Co content due to increasing degree of alloying as the crystallite size remains constant. The durability of the catalyst increases with annealing due to both an increase in the degree of alloying and crystallite size. (C) 2009 Elsevier B.V. All rights reserved.