Sandwich structures with carbon fibre reinforced plastic (CFRP) facesheets are widely used in aerospace and marine structures because they have high strength, stiffness and light weight. However, debonds between the facesheets and the core can reduce greatly the stiffness and the strength of the structures, whilst affecting the vibration behaviours. Hammer impact tests and finite element simulations were conducted to analyse the vibration behaviours of sandwich structures with single and double debonded regions, different matrix modifiers and facesheet stacking orientations. The debonded regions reduced the natural frequencies of sandwich structures, an 80 mm debonded region reduced the natural frequency by 57 %. The natural frequencies in bending modes of the structures with facesheets were more sensitive to debonds; while structures with facesheets were more sensitive in torsion modes. When the debonds were present in the same location for both upper and lower facesheets, there was a greater reduction in the natural frequencies of the bending modes than for other debond arrangements. Reductions in the natural frequency can cause a structure to vibrate at resonance and cause structural failure, therefore understanding of how debonded regions affect the vibration of sandwich structures is critical.