The Charles Beltjens collection of the Sittard-Geleen city archive contains manuscripts of poetry; albums with clippings of poems printed in newspapers, alternated with hand-written lyrical poetry; magazines in which work was printed; works that were (probably) published on his own; newspaper clippings about his life and work; some photographs, and more. The entire collection takes up 64 inventory numbers, 60 of which are on display here. The entire collection fits into a single box file.

In 2004 the collection was expanded with five hand-written poems that were donated by François Beltjens of Tournus, the great-grandson of Charles's brother Frans.