The integrated absorptions of the IR active bands at about 1390 cm-1, 1080 cm-1 and 780 cm-1 were used to determine the content of hexagonal boron nitride h-BN (sp2 bonding state) and cubic boron nitride c-BN (sp3 bonding state) modifications in r.f. sputtered stoichiometric BN films. For the calculation of oscillator number densities in hexagonal boron nitride (h-BN) from integrated absorptions, calibration constants A(h) can be used, which are found to be 1.7 X 10(19) cm-2 for the B-N stretching vibration at 1390 cm-1 and 9.1 X 10(19) cm-2 for the B-N-B bending vibration at 780 cm-1, respectively. It is shown that the ratio of the volume fractions f(c)/f(h) of mixed h/c-BN films can be determined from the ratios of the absorption coefficients alpha(1080 cm-1)/alpha(1390 cm-1) and alpha(1080 cm-1)/alpha(780 cm-1) multiplied by factors of 0.6 and 0. 19, respectively.