When a buckle propagates down an underwater pipeline due to excessive hydrostatic pressure, the permanent plastic deformations and sectional collapse of the tube are restricted to a finite length transition zone. The existing theoretical predictions of the propagation pressure Pp are based on ring models neglecting the axial effects in the transition zone. An analysis of the transition phenomenon has not been reported so far. The present paper analyzes the transition zone in an unconfined propagating buckle using a simple model of the pipe consisting of rings and stringlike generators. The thrust of this paper is toward establishing a basic understanding of the transition phenomenon, necessary to improve the theoretical predictions of the propagation pressure further. Expressions for the length of the transition zone have been derived in terms of the material and geometric parameters of the pipe. The results are compared with limited experimental data kindly provided to us by Kyriakides.