In a method of sealing separate flat lids (16) on to a series of generally rectangular tray-like containers (11), particularly containers which are coated with a polyester such as polyethylene terephthalate for heat resistance, the containers are moved continuously along a line while a lid (16) is positioned with its margin in contact with a peripheral flange (26) on each container, marginal side portions (25) of the lid and/or the side portions of the container flange are bent, e.g. by profiled strips (23, 24) acting as ploughs, to form a V-shaped recess between them, hot air is directed into the recess, e.g. from elongated nozzles (31, 32), to activate adhesive within the recess and to heat the side portions (25, 26), after which the side portions are pressed into contact to seal them together, e.g. by profiled strips (33, 34) and pressure rollers (35, 36). The movement of the containers (11) is thereafter changed, e.g. by turning the line of movement through 90.degree. or rotating each container through 90.degree., so that what were previously the end portions of the lid margin and container flange are presented at each side of the line of movement, and the marginal end portions (42) of the lids are sealed to the end portions (46) of the container flange to complete a peripheral seal between lid and container.