The electrical conductivity produced by $\beta $-ray bombardment and by light irradiation in several diamonds was investigated in the temperature range 150 to 500 degrees K. Considerable uniformity was observed in the behaviour of the specimens; all of them polarized strongly in the dark at room temperature, and polarization could be effectively eliminated by pulsing the electric field while maintaining the $\beta $-flux. Both the counting response and the photoconductivity increased as the temperature was lowered. This was interpreted as being largely due to an increase in the mobility of the charge carriers at lower temperatures, the quantitative relationship being consistent with Hall effect measurements. Accumulation of polarization was reduced by irradiation with red light or by elevation of temperature; at 420 degrees K little sign of polarization remained. From various temperature effects, the thermal depths of the trapping levels in these diamonds were estimated at 0$\cdot $6, 0$\cdot $8 and 1 eV.