We describe a linear succinate-malonate buffer which simplifies mixing determinations in reactor vessels using pH sensors and a conventional stimulus-response technique. The buffer has a linear titration curve between pH 3 and 6, which simplifies mixing determinations due to the direct proportionality between tracer concentration (strong acid or base) and sensor signal(pH). By using the difference between two pH measurements as the tracer signal, the effect of the position of the probes in the reactor is reduced and the initial and final values of tracer signal become zero, which also simplifies the determination of mixing times. Furthermore, the combination of low pH and organic acids in the buffer inhibits microbial growth. The method was tested in three different reactor types : A bubble column, an airlift reactor, both 301, and a conventional 21 stirred tank reactor.