The phase formation of (Bi,Pb)2-Sr2-Ca2Cu3Ox (denoted as Bi/Pb(2223)) in Bi/Pb(2223) silver-sheathed tapes has been investigated using transmission electron microscopy (TEM). In partly reacted Bi2Sr2CaCu2Ox (Bi(2212)) grains, spatial transitions from Bi(2212) layers to Bi/Pb(2223) layers along the basal plane are observed, but they occur very rarely. These transitions cause considerable lattice distortions. Amorphous regions, which correspond to a liquid phase present during the heat treatment, have slightly higher Bi and Pb contents than the adjacent Bi(2212) grains. An analysis of stacking sequences demonstrates that the majority of the layers is surrounded by layers of the same type and that the conversion rate of the Bi(2212)-to-Bi/Pb(2223) transformation decreases rapidly. These observations support the idea that the Bi/Pb(2223) phase primarily forms via a nucleation-and-growth process, which is orientation-controlled by the unreacted Bi(2212) grains serving as a substrate for the growing Bi/Pb(2223) phase.