A thermal microsystem with integrated heaters, pressure and temperature microsensors, has been fabricated to study local temperature and pressure fluctuations occurring in forced convective boiling in microchannels. The observed two-phase flows can be classified into two patterns: oscillating liquid/vapour interface and liquid burst flow; both leading to unsteady temperature and pressure fields. FFT power spectra of the measured signals are correlated with flow visualizations to analyse the two-phase flow modes. The dominant fluctuation frequency of each flow mode increases with input power; and, under similar conditions, the frequency of the periodically oscillating liquid/vapour interface is higher than the dominant frequency of the liquid bursts. Dimensional analysis is performed to derive empirical correlations for the dimensionless fluctuation frequency, Strouhal number, for both flow patterns.