Masonry arches are typical structural elements of existing masonry constructions. Many existing masonry buildings and bridges in Europe, and indeed in many other parts of the world, are quite old and consequently may show defects and/or degradation. Such defects and degradations in masonry constructions may be caused by different factors, both environmental and human. One of the most common defects that develop over time in masonry structures involves local or uniform reduction in the thickness of the structural elements. This paper uses limit analysis to examine the influence of local thickness reductions on the seismic capacity of masonry arches: in terms of collapse acceleration and changes in collapse mechanism