Purpose: The AAPM is currently working to provide an update/addendum to the TG-51 dosimetry protocol. As part of this work measured and calculated kQ factors for a range of thimble-type ionization chambers not listed in TG-51 were obtained. The aim was to investigate the accuracy of the calculations and obtain objective evidence on the performance of these chambers for reference dosimetry. Method and Materials: The same formalism and computer programs as employed in TG-51 were used to calculate kQ factors. As well as 0.6 cm3 reference chambers, factors were calculated for scanning and micro chambers from the major manufacturers. Measurements were made using the and Elekta Precise linac facilities at the National Research Council of Canada. The aim was to characterize the chambers over the range of energies applicable to TG-51 and determine whether the chamber met the requirements of a reference class instrument. Chamber settling, ion recombination and polarity were investigated and absorbed dose calibration coefficients were obtained for and 6, 10 and 25 MV photon beams. Results: As might be expected, 0.6 cm3 thimble chambers showed the most predictable performance and experimental kQ factors were obtained with a relative uncertainty of 0.1%. The performance of scanning and micro chambers was somewhat variable. Some chambers showed very good behaviour and gave reasonable agreement between measured and calculated kQ factors but others showed anomalous polarity and recombination corrections that require further investigation. For the well-behaved chambers, agreement between measured and calculated kQ factors was within 0.4%. However, for some chambers differences of nearly 1% were seen that may be related to the recombination/polarity issues. Conclusion: Experimental and calculated kQ data have been obtained for a wide range of thimble chambers that can be used by the AAPM and clinical users in choosing suitable detectors for reference dosimetry.