A novel technique for studying paramagnetic defects in insulators or s
emiconductors acting as positron traps is presented. The spin polariza
tion of the paramagnetic electrons of such centres can be determined b
y investigating the annihilation of spin-polarized positrons, e.g., by
Doppler-broadening or positron-lifetime measurements. The viability o
f this technique is demonstrated by measuring the temperature dependen
ce of the polarization of the paramagnetic electrons in F-centres in K
Cl in high magnetic fields (+/-4.5 T).