We have employed the constant photocurrent method at 120 K to study the low temperature creation and annealing kinetics of light induced metastable defects in a good quality undoped a-Si:H. Infrared selfquenching influences the CPM spectra taken at 120 K and therefore optical absorption coefficient alpha(CPM) measured at this temperature underestimates the defect concentration. However, relative concentration changes of the metastable defects in various light exposed or annealed states can be deduced from these measurements. We have obtained satisfactory results for the light induced defect creation kinetics at 120 K, and from a systematic annealing experiment we have deduced the activation energy distribution function for the metastable light induced defects created at 120 K. The result is a broad, resembling a two component distribution of defect annealing activation energies. Comparing this distribution with the one obtained by Q. Zhang, II. Takashima, J.H. Zhou, M. Kumeda, T. Shimuzu [Phys. Rev. B 50 (1994) 1551] we have come to the conclusion that defects with low annealing activation energies are firstly created at the earlier stage of the illumination. (C) 1999 Elsevier Science B.V. All rights reserved.