This paper presents experimental and analytical studies on the causes of differences between reinforced concrete (RC) and steel-reinforced concrete (SRC) column behaviour with a focus on moisture diffusion. The results of experiments on the long-term behaviour of RC and SRC columns have shown that the shortening of SRC columns tends to be overestimated by the methods generally used to predict the shortening of RC columns. Moisture diffusion coefficient and surface factor of the concrete used in the SRC column specimens were determined from experiments and an analysis on moisture diffusion was conducted to determine the progress of moisture diffusion in the SRC column specimens. The analytical results indicated that the progress of moisture diffusion in SRC columns is quite different from that in RC columns. Analyses on differential drying shrinkage and differential drying creep were also performed by analysing the progress of moisture diffusion at each point in an SRC column section. The results indicated that obstruction of the moisture diffusion route causes a reduction in both drying shrinkage and drying creep in the SRC columns. The differential moisture distribution should therefore be taken into account in order accurately to predict SRC column shortening.