In recent years, there has been rapid progress in the development of solid oxide fuel cells (SOFCs) to operate in the temperature range of 600 to 800ЊC. 1-3 This significantly opens up opportunities in the selection of a wide range of materials for electrodes and stack components for planar-type SOFCs. In particular, a low operating temperature would enable the use of metallic interconnect materials. 4 Metallic interconnects have higher electronic conductivity, higher heat conductivity, and better machinability when compared to LaCrO 3 based ceramic interconnect materials. However, chromiaforming alloys such as stainless steel generate volatile Cr-containing species at high temperatures in oxidizing atmospheres. 5 Without effective protective coatings, this can lead to rapid degradation of the SOFC performance due to chemical interaction of Cr species at the Sr-doped LaMnO 3 (LSM) cathode side. 6-8 The degradation mechanism is considered to be dominated by an electrochemical reduction of high valent vapor species of chromium [CrO 3 and Cr(OH) 2 O 2 ] to solid phase Cr 2 O 3 in competition with the O 2 reduction reaction, followed by the chemical reaction with LSM to form (Cr,Mn) 3 O 4 phases at the three phase boundary (TPB) region. 7-10 The increase in cathodic polarization was found to be related to the amount of Cr at the cathode/electrolyte interface region. 9 Thermodynamically, deposition of Cr species can take place either by electrochemical reduction and/or by chemical dissociation. 10 Quadakkers et al. 11 studied transportation and deposition processes of Cr species on LSM, LaCoO 3 and Sr-doped LaCoO 3 (LSC) films coated on aluminaforming and chromia-forming alloys at 950ЊC. In the case of the chromia-forming alloy, Cr species were deposited over the whole width of the coating, forming spinel phases of MnCr 2 O 4 for an LSM coating and CoCr 2 O 4 for an LSC coating. However, despite the potential application of chromia-forming alloys as interconnect materials in SOFCs, the mechanism and kinetics of deposition processes of Cr species at LSM electrodes are far from clear.We have recently investigated the initial deposition processes of Cr species at LSM electrodes, and the results indicated that Cr deposition at the LSM electrode/electrolyte interface region may not be dominated by electrochemical reduction of high valent Cr species. 12 In this paper, we report in detail the results of deposition processes of Cr species at LSM electrodes under various polarization and nonpolarization conditions. New deposition mechanisms of Cr species at the LSM electrode/yttria-stabilized zirconia (YSZ) electrolyte are presented, and the driving force for the deposition of Cr species is discussed. The effect of Cr deposition and Cr species on the O 2 reduction reaction on LSM electrodes was given in the second part of this series. 13Experimental LSM with the composition La 0.72 Sr 0.18 MnO 3 (LSM) was prepared by coprecipitation, followed by sintering at 1000ЊC in air. The X-ray diffraction (XRD) pattern showed perovskite ...