In the present study the effect of using magnetic water treatment in order to reduce carbonate deposit during membrane distillation was investigated. Tap water containing 2.16 mmol HCO3- dm(-3), which was additionally enriched with bicarbonates (alkalinity 2.72 mmol HCO3- dm(-3) and 3.61 mmol HCO3- dm(-3)) was used in the study. To generate a magnetic field a commercial device, called the magnetizer, was used. The magnetic field originated from two S-S type permanent magnets, 0.1 T each. For comparison purposes the membrane distillation with/without a magnetic pre-treatment of the feed were conducted. The distillation process proceeded at 358 K. When the feed was warmed up, large amounts of CaCO3 deposit were formed inside the experimental installation. SEM-EDS and XRD were used to investigate the chemical composition and morphology of the precipitate. The precipitate was also forming on the membrane surfaces, which resulted in a decrease of modules efficiency. It was found that as a result of magnetic pre-treatment larger crystallites were formed and the deposit was more porous. For this reason a decrease in the permeate flux was significantly smaller when the magnetizer was used. The CaCO3 deposit was found to be mostly in the form of calcite in each investigated case. (C) 2011 Elsevier B.V. All rights reserved.