Undoped Bi-2223 samples were prepared using a conventional solidstate reaction method. Doping of Au in Bi-2223 was carried out by means of diffusion from an evaporated Au film on pellets. We have investigated the effect of Au diffusion and diffusion-annealing duration on the microstructure and superconducting properties of Au-doped samples by performing x-ray diffraction (XRD), scanning electron microscopy (SEM), dc resistivity and critical current density measurements. Gold diffusion in Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy has been studied over the temperature range 500-800 degrees C using the technique of successive removal of thin layers and the measurements of lattice parameters from XRD patterns at room temperature. The diffusion doping of Bi-2223 by Au causes a significant increase of the lattice parameter c by about 0.19%. This observation is used for calculation of the Au diffusion coefficient in Bi-2223. The Au diffusion coefficient decreases as the diffusion-annealing temperature decreases. The temperature dependence of the Au diffusion coefficient in the range 500-800 degrees C is described by the relation D = 4.4 x 10(-4) exp(-1.08 eV/ k(B)T). Au doping of the sample increased the critical transition temperature and the critical current density from 100 +/- 0.2 to 104 +/- 0.2 K and from 40 to 125 A cm(-2), in comparison with those of undoped samples. The critical transition temperature and critical current density of Au-doped samples increased with increasing diffusion-annealing time from 10 to 50 h. Au doping of the sample also improved the surface morphology and increased the high-T-c phase ratio. The possible reasons for the observed improvement in superconducting properties of the samples due to Au diffusion are discussed.