We report on measurements of combustion-relevant fuel properties for on-line, feedforward control with small, rugged and fully compensated microsensor-based systems. Such silicon microstructure sensor systems have been demonstrated to determine gaseous and liquid fuel properties such as stoichiometric oxygen demand, octane number, heating value, density and other properties of interest. The measurement approach consists of a three-step process: (1) Measurement of changes in electrical quantities when the sensing elements come in contact with the fluid, (2) Conversion of these quantities into primary sensor outputs, yi, such as thermal conductivity, specific heat, temperature and pressure, and (3) Correlation between these and the properties of interest, Y(yi). By coupling this property sensor to an equally rugged and small thermal flow microsensor, millisecond-range response time signals of mass or volume flow, or stoichiometric oxygen demand rate are provided for feed-forward control, without exposing any sensor to harsh exhaust gas environments. Having presented results with gaseous fuels elsewhere, we update these here but concentrate on the determination of octane and cetane number of liquid fuels. Analysis results show that the correlations between these combustion performance properties and physical fuel properties are as good as the ones between octane and critical compression ratio or between cetane and ignition delay. However, all those correlations appear to be limited presently by the accuracy or at least consistency of available data, which are needed for calibration of the sensor system. In checking the temperature dependence of one of the correlations for octane, we found the system output to shift by 15% when using hexadecane as a reference fuel, but only by 1% with iso-octane as reference, for a 10 degree(s)C shift in temperature.