Abstract Ca 2+ sensors of high sensitivity and stability can be fabricated by coating Si/SiO 2 devices with specially synthetisized Langmuir-Blodgett (LB) membranes. The Ca 2+ sensitive LB-material is an amphiphilic copolymer containing covalently bound ionophoric groups. A thin cover layer of hydrophobic rod-like LB-polymers increases the sensor stability in contact with electrolytes. The membrane show a high Ca 2+ sensitivity of 23 mV/pCa in the range 10 −4 to 1 mol/l and a small baseline drift of less than 1 mV/day over a period of at least 3 month. The sensor response after concentration changes was very fast with a response time of less than 20 s. The sensor can be produced at low cost. Miniaturization is possible. Applications are in the fields of measuring water hardness or as a cheap substitute for conventional Ca 2+ electrodes.