This study investigates propagation buckling of subsea pipe-in-pipe (PIP) systems under hydrostatic pressure. Unlike in previous studies, PIP systems consisting of carrier pipes with a diameter-to-thickness (Do/to) ratio in the range 26–40 are examined here. Experimental results from ring squash tests (RSTs), confined ring squash tests (CRSTs), and hyperbaric chamber tests are presented and compared with a modified two-dimensional (2D) analytical solution and with numerical results using three-dimensional (3D) finite-element (FE) analysis. The comparison indicates that the proposed modified analytical expression provides a more accurate lower-bound estimate of the propagation buckling pressure of PIP systems compared with the existing equations, especially for higher Do/to ratios. The novel RST and CRST protocols proposed for PIP systems give lower-bound estimates of the propagation pressure. The FE analysis outcomes demonstrate that the lengths of PIP system transition zones are almost twice the corresponding lengths in single pipes. New modes of buckling are discovered in the hyperbaric chamber tests of PIP systems with Do/to=26.