High-resolution gamma radiation spectroscopy (using the HPGe detector) was used to study the potassium content by recording 1461 keV gamma radiation from the radioactive decay of the 40K isotope (about 0.012% of the isotopic composition of potassium in the Earth’s crust). The measurements may be realised on samples in hermetically sealed containers and are sensitive to the presence of potassium only. The detector was placed in thick lead shields, which improved the sensitivity of the method. The measured intensity of 1461 keV radiation from samples containing KCl solution of various concentrations from 0.002 to 0.1 mole/litre was used to obtain the calibration line. The sensitivity of the method was determined as 0.7 mmol/litre, which corresponds to 40K activity of 0.9 Bq/litre. A study of the potassium content in 5 ketchup samples was carried out, confirming a similar potassium content in the samples differing in the production date and the degree of spiciness. About twice the potassium content was found in the “premium” product – probably more tomatoes were used in its production per kilogram of product. Apart from changes in the intensity of 1461 keV radiation, no increase in the intensity of gamma radiation compared to the background was found for any of the ketchup samples.