Among the treasures missing were rosary beads carried by Mary Queen of Scots at her execution in 1587 as well as rare gold and silver treasures (see below).
The reward is offered on the basis that any of the missing items are returned in their original, undamaged, condition, subject to specific conditions, and anyone with information is asked to contact Simon Jones at loss adjusters Quadra Claims Services, email info@quadraclaims.co.uk.
The police continues its investigation and released photos of two ladders used by the thieves.
One of the ladders that Sussex Police is looking for information on.
Det Insp Alan Pack of Sussex Police said: "The ladders have clearly been well used over some years. The long ladder has some distinctive black and yellow paint splashings and each has some worn labelling.
"We hope that someone in the decorating or building trade, or maybe someone who just had them at home, will realise that they are now missing them, and will contact us.”
Anyone with information on this incident should call 101 quoting Operation Deuce or call Crimestoppers anonymously on 0800 555 111.
Missing items from Arundel Castle
Mary Queen of Scots Rosary Beads;
Seven gold / silver-gilt coronation cups (George II, George III, George IV, William IV, George V, George VI and Elizabeth II);
Gold Earl Marshal’s baton;
Gold and enamel baton;
Other miscellaneous items including 10 silver-gilt Apostle Spoons, a silver-gilt casket with hunting and fishing scenes, and a silver-gilt mug.
