Effect of temperature on ultrasonic attenuation in BCC structured (CsCl-type) thallium halides (TlCl and TlBr) have been investigated in a wide temperature range 50-500 K for longitudinal and shear modes along [1 0 0], [1 1 0] and [1 1 1] directions of propagation. Starting with nearest neighbour distance and repulsive parameter and taking interactions up to next nearest neighbours, second and third order elastic moduli have been evaluated, which in turn have been used for evaluating thermal relaxation time, Gruneisen numbers, acoustic coupling constants, ultrasonic velocity and ultrasonic attenuation. Ultrasonic attenuation in these bcc structured crystalline materials has been found less than the fcc crystalline materials. The results have been discussed and compared with available data. (C) 2009 Elsevier B.V. All rights reserved.