An in situ, easy, and versatile method to decorate a polymer substrate with uniformly spaced centimeter- long arrays of 5 - 10 nm palladium nanoparticles is reported. The electrochemical deposition of palladium nanoarrays is described as well as results from various microscopic and spectroscopic characterizations, such as transmission electron microscopy, scanning electron microscopy, optical microscopy, and electron spectroscopy for chemical analysis for their size, morphology, chemical constituent, and chemical state, respectively. The Pd nanoarrays were synthesized using a special kind of active template from an electrolyte solution of ionic strength 0.10626 mol g(-1). The effect of curvature on the active template is discussed briefly. Understanding of the preferentially deposited Pd nanoparticles to form nanoarrays longitudinally under controlled conditions can lead to the preparation of low- cost, lightweight, reliable hydrogen sensors for safety monitoring applications. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.1945667] All rights reserved.