A SAW based tyre pressure sensor assembly comprising a body having a tubular stud 1 formed on an end thereof which is engageable in an inflation tube formed in a tyre inflation valve 3 so as to fasten the body to the valve 3. A through passage extends through the stud 1 into said body and is in fluid communication with the atmosphere surrounding the assembly by means of apertures 2 in the sides of the body, thereby providing a fluid coupling between an inflation duct of the inflation valve 3 and the atmosphere surrounding the body. A SAW based pressure sensor is mounted within said body so as to be in fluid communication with the atmosphere surrounding the body, and an antenna 4 is connected to the sensor to enable remote interaction therewith. A grounding plate 5 connected to the antenna is sandwiched between the assembly and has arms which engage against the inner surface of the wheel so as to enhance the sensor antenna earth.