The present invention relates to a fire resistant glazing unit (10), and in particular to novel curable gelling compositions which may form a fire-resistant hydrogel interlayer (3) between the glass panes (1) of the glazing unit (10). The gelling compositions of the invention comprise 5-40% w/w of a curable hydrogel-forming component (typically an acidic vinyl monomer such as acrylic acid or its metal salt); 20-60% w/w of one or more salts (e.g. magnesium salts); 40-90% w/w of an aqueous vehicle; and the gelling compositions have an acidic p H between p H 1 and 7. Such gelling compositions, when cured to form a hydrogel interlayer, provide excellent fire-resistant properties and also an excellent radiation barrier in the event of a fire.