A field-portable photometer for potassium determination with disposable sensors has been developed. It can be applied to routine water and beverage analysis. The disposable sensor is based on ionophore-chromoionophore chemistry. A colour change in the sensing film is detected by measuring the transmitted intensity with a solid state photodetector. Optical excitation at 660 nm is emitted by a light-emitting diode (LED). Negative feedback for LED bias and thermal correction were included to improve system stability. Additionally, a measurement procedure is presented, characterized and validated for in situ photometer use and real-time results. This simplified procedure is based on prior preparation of the disposable sensor in its acidic form and on the use of an absorbance ratio as analytical parameter. The only requirement for analysis is prior equilibration with a buffered sample solution for 3 min and absorbance measurement before and after equilibration. Good sensitivity in the concentration range 5 muM to 100 mM and very good repetitively and stability were achieved that are comparable to those obtained with bulkier analytical instrumentation. Given the compact size, low weight, rapid response and low energy requirement of the electronic photometer developed here, this measurement system is suitable for potassium determination in the field.