A wide range of materials has been used for gap-filling in wooden objects but very little information exists in the literature on their suitability. Few studies have been conducted on the physical properties of gap-filling materials or their long-term effect on the substrate. In this study 33 gap-fillers were tested for their handling properties and their response to applied stress. Fill materials were first subjected to stress-strain analysis in a compression tester. They were then evaluated for their response to compressive forces when inserted in closing checks (cracks) in swelling wood responding to high relative humidity. The results of the study showed that two types of fill can be acceptable depending upon the type of wood to which they are applied and the environment. These are, firstly, fillers of very low modulus of compression, such as waxes, balsawood, and a silicone rubber/microballoon mixture, which are suitable in most circumstances; and secondly, strongly adhesive fillers of high modulus of compression, such as an epoxy resin/microballoon mixture, which can be used in applications where the wood will be maintained in a moderately stable relative humidity.