Using bright-field and energy selective x-ray analysis techniques in transmission electron microscopy, we have studied the effect of Ca doping on the microstructural and superconducting properties in DyBa₂Cu₃O_y/Ca_z (z = 0, 0.05, 0.1 and 0.2) ceramics, i.e. for the case when Ca was added in excess to the stoichiometric 123 composition. It was found that in this case approximately half of the added Ca amount entered the 123-lattice. Thus, in the composition with nominal z = 0.1 (0.77 at%) average Ca concentration in the grains was about 0.4 at%. On the basis of measured atomic concentrations and 123 stoichiometry of grains, it was concluded that Ca predominantly substituted Dy in the bulk for all levels of doping (0 ≤ z ≤ 0.2), although for z = 0.2 the rate of substitution for Ba also became pronounced. This substitution of Dy³⁺ by Ca²⁺ seems to explain the observed decrease in T_c from 92 K in an undoped sample to 74–77 K for compositions with z = 0.1–0.2. Ca segregated in grain boundaries (GBs). The concentration of Ca in most 'clean' (secondary-phase-free) GBs was 1.5–3.5 times higher than in the adjacent grains. Ca segregated in a narrow region (d ≤ 5 nm) around the GB. In the majority of GBs, Ca substitution for Dy was favoured, although with increasing the doping level from z = 0.1 to z = 0.2 the rate of the substitution for Ba increased. This substitution for Dy most probably explains the obtained dramatic increase of J_c at T ≤ 0.8 T_c in the Ca-doped samples. Ca segregation in GBs is the most probable reason for the superconductor–normal-metal–superconductor (SNS) behaviour of the GB network in Ca-doped samples. Ca doping led to a higher proportion of clean GBs, in the sample with z = 0.1, that could be responsible for a slower decrease of J_c with magnetic field in this sample.