Continuously tunable microcavity devices based on III/V semiconductors are key elements for future dynamic wavelength division multiplexing (WDM) systems. In this paper we present technological aspects as well as device results of surface micromachined InP-based tunable Fabry-Perot (FP) filters. Using air-gap technology and electrostatic actuation we achieve a wavelength selectivity of 0.5 nm full-width at half maximum (FWHM) for the multi-air-gap devices. The entire tuning range is 62 nm for an actuation voltage of 14 V. Single air-gap devices with solid Bragg mirrors show similar tuning characteristics and inferior values in terms of wavelength selectivity (FWHM = 10 nm). However, these devices are well suited for the monolithic integration of a photodetector.