  We present measurements with a GEM based TPC prototypes and the TimePix CMOS
ASIC as charge collecting anode. Tests with a small prototype were performed in
a 5 GeV electron beam at DESY. Different GEM orientations and geometries were
tested. It is shown that the point resolution for short drift distances is
better than 20 micrometer. The achieved time resolution is better than 10 ns at
100 MHz clock frequency. Furthermore, experimental studies with an enlarged
pixel size are addressed. To study the performance for longer drift distances a
chamber with 26 cm drift length is operated in Bonn. Using also GEM and TimePix
for the readout, data with cosmic muons and a Sr90 source are recorded. The
spatial resolution and cluster properties are studied in dependence on the
drift distance.
