A novel vapor sensor is presented comprising a capacitive porous silicon (PS) transducer and phase-detection electronics. Sensing of ambient vapor concentration by adsorption-diffusion-condensation kinetics of vapor molecules, by the PS layer leading to a change of its dielectric constant is modeled with an effective medium approximation (EMA). A simple but precise phase sensitive electronic circuit has been developed. This detects any change of phase of a sinusoidal signal transmitted through the PS dielectric and correlates with ambient vapor concentration. The sensor is tested in combination with the phase detection circuitry in case of relative humidity. Excellent linearity over the entire range of relative humidity is achieved. Experimental results show a resolution better than 0.1% and accuracy 2% (near the transition region) and better than 0.1% (otherwise). The response time is less than 10 sec. with good stability.