Polycrystalline perovskites compounds HoMn/sub 1-x/Co/sub x/O/sub 3/ (x=0--0.8) have been prepared by conventional solid-state reaction. Here, we used far-infrared (FIR) spectroscopy to study infrared active phonon modes and present a comparative analysis of infrared transmission spectra of polycrystalline HoMn/sub 1-x/Co/sub x/O/sub 3/ (x=0--0.8). The data indicated that phonon modes significantly changed with increase of cobalt doping level. Four main bands were assigned as external, torsional, bending and stretching bands. The external vibration energy remain same at /spl omega//sub 1//spl sim/190 cm/sup -1/ for Co doping x/spl les/0.5 and shift to higher energy for x>0.5. Torsional and bending bands exhibit splitting. The stretching band is at 600 cm/sup -1/ for all samples, but the bandwidth is reduced as Co doping increased. The transmission spectrum of HoMn/sub 4/5/Co/sub 1/5/O/sub 3/ was analyzed to obtain the spectrum of optical density. The minimum number of oscillators to obtain a reliable fit is 5 by using a sum of noninteracting harmonic oscillators.