Isotherm hysteresis and pore-blocking effects of trapped molecules on adsorption dynamics is studied here, using the iodine-carbon system in the 300-343 K temperature range. It is found that a portion of the iodine is strongly adsorbed, and does not desorb, even over very long time scales, while the remainder adsorbs reversibly as a homogeneous monolayer with a Langmuirian isotherm in mesopores. The strongly adsorbed iodine appears to adsorb in micropores and at the mesopore mouths, hindering uptake of the reversible iodine. The uptake data for the adsorption and desorption dynamics of the reversible part is found to be best explained by means of a pore mouth resistance control mechanism. It is concluded that the dynamics of the adsorption and desorption at the pore mouth is important at early stages of the process.