New metal-oxide type pH sensors with a direct solid state membrane contact have been developed and tested. Performance was demonstrated at close to ambient temperature with thin, 0.3-0.5 mm stabilized cerium oxide Ce0.80Sm0.20O1.9-x membranes. The pH sensors responded reproducibly to the acidity changes of the analyte in the range of pH 3-10. The response time is 1 to 2 minutes for 95% response. pH sensors with thin ceria membrane can be used also to follow acid-base titrations.