Summary: In the present work, we examined the head-on collision of solitary waves in shallow water theory, through the use of extended Poincare-Lighthill-Kuo (PLK) method based on the combination of reductive perturbation method with strained coordinates. Motivated with the result obtained by A. E. Ozden and H. Demiray [“Re-visiting the head-on collision problem between two solitary waves in shallow water”, Int. J. Non-Linear Mech. 69, 66–70 (2015; doi:10.1016/j.ijnonlinmec.2014.11.022)], we introduced a set of stretched coordinates that include some unknown functions which are to be determined so as to remove secularities that might occur in the solution. By expanding these unknown functions and the field variables into power series in the smallness parameter \(\epsilon\), introducing them into the field equations and imposing the conditions to remove the secularities, we obtained some evolution equations. By seeking a progressive wave solution to these evolution equations, we determined the speed correction terms and the phase-shift functions. The result obtained here is exactly the same with found by Ozden and Demiray [loc. cit.], wherein the analysis employed by C. H. Su and R. M. Mirie [J. Fluid Mech. 98, 509–525 (1980; Zbl 0434.76021)] is utilized.