We report the "rst measurements of FIR and MIR re#ectivity of single-crystal monoclinic antiferromagnetic copper monoxide (CuO) in a wide temperature range (7}300 K) when b-axis and ac-plane polarized modes are completely separated and excited in transverse geometry. Previously reported softening of the A# # mode at the NeH el transition is con"rmed and found to be up to two times more signi"cant (about 10%). For the "rst time an emergence of several new IR-active lines at low temperatures is observed. The highest frequency new mode at 690 cm## strongly hardens and strengthens with cooling. The data are discussed in terms of strong spin}phonon coupling and possible charge disproportionation at low . # 2000 Elsevier Science B.V. All rights reserved. Keywords: Charge disproportionation; Infrared spectroscopy; Magnetic ordering; Spin}phonon interaction 1.