ABSTRACT Infrared absorption spectroscopy has been used to study the vibrational modes in various borate crystals, the electronictransitions of Nd3 ions in NYAB, and the stretching vibration of hydroxyl ions in CLBO ciystals.Keywords: borates, CLBO, YAB, FTIR spectroscopy, vibrations, Off ions 1. INTRODUCTION Borate crystals possess excellent optical and non-linear optical (NLO) properties."2 Owing to the occurrence of boron in three or four-fold coordination a variety of borates can form, combining the tngonal B03 and the tetrahedral BO4 molecular units. Cyclic borates, like f3-BaB2O4 (BBO), LiB3O5 (LBO), Li2B4O7 (LTB), CsLiB6O10 (CLBO), and someorthoborates with huntite structure, like YA13(B03)4 (YAB) and NdY1Al3(BO3)4 (NYAB) have been grown recently. Inthe present contribution we focus our attention on the YAB crystal which is an excellent non-linear optical laser host, andon the CLBO which exhibits superior ultraviolet NLO properties. YAB crystals are built up from the trigonal (BO3)3anion group while CLBO crystals consist of the (B307)5 group containing two trigonal and one tetrahedral molecular unit.Beside the vibrational properties of the various molecular units the energy levels of Nd3 ions and the stretching vibration