The infrared absorption spectra of phosphorus acids RZPOOH (R=H, CH3, CH2Cl, CH2I, C6H5, C3F7, CH3O, C4H9O) have been recorded in the gas phase and in the solid state in the range 4000-1000 cm(-1). The equilibrium between monomers and dimers of R2POOH was studied in the gas phase between 400-650 K. The broad absorption band of the v(OH) stretching vibration of the cyclic dimer in the region 3600-900 cm(-1) is registered in the gas phase spectra at 400-600 K and in thin films spectra at 90-300 K. In all cases the band has the characteristic ABC structure, typical for spectra of complexes with strong hydrogen bond in the crystal phase and solution. The intensity and the first and second spectral moments of this band were determined. It was found that, on passing from thin films at low temperature to the gas phase at high temperature, the redistribution of intensity from the low frequency wing to the high frequency wing takes place. Despite significant differences of dimerization enthalpies (20-60 kcal/mole), the spectra of all acids have similar structure and close values of spectral moments; although the values of intensities are different.