Solar thermal conversion systems can be used for driving a Multi-Effect Distillation (MED) unit, namely: parabolic trough collectors, compound parabolic concentrators, evacuated tube collectors, flat plate collectors and salinity-gradient solar ponds. This paper deals with an experimental test campaign of a Multi-Effect Distillation (MED) unit installed at the Plataforma Solar Almeria (PSA-CIEMAT) operated out of its nominal working conditions in order to provide experimental information required for design criteria and for the analysis of control strategies in solar MED plants. The MED plant (SOL-14) assessed is a conventional forward-feed unit with preheaters, 14 effects and nominal capacity of 3 m(3)/h. The experimental test campaign of the SOL14 plant conducted at the PSA proves that the PR exhibits low deviations within the range of working conditions analysed. The SOL-14 unit makes an efficient use of the energy consumption even when it is operated out of the nominal working conditions. The effects of part load operation are higher on distillate production than in PR. On the contrary, at working conditions over nominal, the PR suffers from higher deviation than distillate production, which remains unchanged within the range analysed. (C) 2011 Elsevier B.V. All rights reserved.