Abstract Potassium release from weathering of soil minerals may support the K nutrition of crops for many years. However, when soils become exhausted, the response to K fertilisation may be limited due to its fixation in non-exchangeable forms, reducing the efficacy of K fertilisation. The present study examines the role of soil mineralogical composition on the K fixation characteristics of soils developed under a Mediterranean type of climate. Nine soils derived from different parent materials were collected in several regions of Portugal. Soil properties were determined, and clay, silt and fine-sand fractions were studied by X-ray diffraction. Potassium fixation was determined after the soil samples had been treated with increasing rates of K application. The amount of K fixed was obtained by difference, measuring the amount of K remaining extractable by ammonium acetate. The soils under study showed a relatively high K fixation capacity, varying between 30 and 80% for an application rate equivalent to 800 kg K ha−1. Soils with high K fixation capacity were derived from gabbros, gabbrodiorites and quarzdiorites, and had relevant amounts of vermiculites and/or interstratified mica-vermiculite minerals, either in the clay or in the silt and fine sand fractions. Soils rich in calcium carbonates also fix high amounts of K. These soils contain mica-illite minerals and are rich in some of the above-mentioned minerals in the clay and silt fractions. While K fixation capacity is normally assumed to derive from minerals in the clay fraction, the results of this study show that vermiculites and/or interstratified mica-vermiculites present in the silt and fine sand fractions can contribute a significant proportion of the total K fixation capacity and, thus, these size fractions should also be included in any assessment of K fixation capacity.