The composition and sintering characteristics of deposits of alkali metal salt particles formed in-situ in a kraft recovery boiler were investigated. The deposits were formed on air-cooled probes inserted into the convective gas passages of an operating kraft recovery boiler. Their microstructure was characterized by scanning electron microscopy. The results show that, in regions of the boiler where the alkali metal salt particles are no longer molten, two types of deposits can form. The first is composed almost entirely of submicron aerosol (fume) particles. The second contains a mixture of fume and larger particles, with dimensions typically 5 to 70 mum. Two types of larger particles were identified: spherical particles of solidified ash from combustion of black liquor, and irregular chunks from upstream deposits disintegrated by soot blowing. The structure of deposits formed from fume particles was one of branched dendritic growths. The deposits as formed were relatively low density. Sintering proceeded via growth of nodes and thickening of branches, but with slow densification of the deposits. All of the deposits observed appeared to harden via sintering of the much finer fume particles. The larger size particles, when present, appeared to be bound in the deposits by the matrix of finer particles that was produced as the finer particles sintered.