In this contribution capillary type thermal mass flow sensors based on the thermo-transfer principle were applied for monitoring esterification reactions in residence time micro-reactors. A novel micro-reactor with an integrated sensor network was developed that enables the inline analysis of six stages of chemical conversion in parallel. The device was optimized for residence times in the range of several tens of minutes. Conversion can be monitored by means of changes in the isobaric heat capacity. Methods for the compensation of offsets and flow pulsation in the linear operation range of the sensor were developed. The novel micro-reactor was used for studies on the homogeneously catalyzed synthesis of butyl acetate. The phase separation at a certain residence time could be successfully detected. The formation of aggregates/plugs with different phases led to oscillations in the measurement signal. Insofar, robust sensors emerge as low cost tool for the inline analysis and control of esterification reactions. (C) 2010 Elsevier B.V. All rights reserved.