The relationship between microstructures and hydrogen absorption-desorption properties described in terms of pressure-composition (P-C) isotherms has been investigated for binary and Co- and Al-added LaNi5-based alloys as a function of the number of hydrogen sorption cycles. From microstructure observations, factors determining absorption pressures are deduced and variations of P-C isotherms with cycle number for these three LaNi5-based alloys are discussed.