Subacute rumen acidosis (SARA) of ruminants is an important factor in terms of animal health, especially in high yielding cattle. In order to find an accurate method to determine the ruminal pH-value, three methods using eight rumen cannulated cattle were compared. An indwelling measuring unit (sensor) was used for continuous measurement of the ruminal pH-value. These results were compared to the pH-values of samplings via rumen fistula and to the results of samplings via oral stomach tube. Due to the different rations, mean pH-value of trial 1 (average of all methods) was 6.64 +/- 0.37 (hay ad lib., 2 kg concentrate/ animal) and 6.24 +/- 0.36 for trial 2 (75% maize silage, 1 kg hay, 2 kg soybean meal). In trial 1 no statistically significant differences between all methods could be observed. Under more acidic ruminal conditions of trial 2 all methods differed significantly (p < 0.05). In the lower pH-range of trial 2, a difference of 0.32 that data can be collected continuously. The sensor system was evaluated by a comparison with standardized pH-dilutions (pH 4, pH 7). The sensor system has proven to be an accurate and reliable instrument (r = 0.9984) and it represents an innovative system for answering scientific questions in terms of rumen physiology and rumen pathology.