Experimental results obtained from the study of 12V, 40Ah, valve regulated lead/acid (VRLA) batteries indicated that reduction in capacity during cycling can be attributed to changes in the positive electrode. Corrosion layer (CL) growth has been identified as a key factor that can lead to a reduction in performance and eventual battery failure. In this paper, a selection of results concerned with the positive grid CL are presented. These can be used to suggest ways of improving the present positive electrode design and also to outline some of the different types of defect that form during operation. In addition, reduction in oxygen concentration through the thickness of the layer towards the grid surface was detected indicating an increase in resistivity and thus internal resistance of the cell.