The ξ-carbide, also described as Fe2MoC and MaCb, has been identified at ferrite grain boundaries in 1Cr-0.5Mo steels exposed to elevated temperatures (500-530°C) for prolonged periods (65,000-170,000 hr). The structure and composition of the phase have been characterized using electron microdiffraction and energy-dispersive X-ray spectroscopy (EDXS) respectively. Analysis of microdiffraction patterns supports the proposal that the ξ-carbide has a monoclinic unit cell, rather than the orthorhombic unit cell first proposed. The ranges of metallic element concentrations in the ξ-carbide (i.e. 48-59 at.% Fe, 22-29 at.% Mo, 7-16 at.% Cr, 4-7 at.% Mn and 3-6 at.% Si) are unique compared to those of other carbide precipitates identified in the 1Cr-0.5Mo steels, which means that the ξ-carbide may be identified rapidly on extraction replicas using qualitative EDXS.