Portable power systems based on direct methanol fuel cells (DMFC) have to provide power in various environmental conditions: it is advantageous for such a power-supply system to be autonomous, i.e. able to operate without water refills for the methanol solution. It is shown that system autonomy depends mainly on environmental humidity, condenser temperature and air excess ratio: this result is valid in general for any DMFC, as cell parameters have only a marginal role. The environmental conditions in which a portable DMFC system may be autonomous are considered, delineating a feasibility envelope. Two methods are proposed to extend this envelope: operating with a diluted methanol reservoir, which improves the autonomy of the system only marginally and at the cost of a large loss in energy density, and system pressurisation, which delivers a more significant improvement in autonomy properties, but at the cost of system efficiency and simplicity. (C) 2010 Elsevier Ltd. All rights reserved.