The activity of iodine‐modified MgO in transfer hydrogenation of acrolein with alcohols can be significantly higher than that of unmodified MgO. Data from literature and our previous studies indicate that the initial source of a halide influences the surface properties and hence the activity of the obtained catalytic system. The aim of this paper is to present a systematic study on the influence of the initial source of a halide on the activity and surface properties of MgO‐based catalysts. For the sake of comparison, results for an MgO sample subjected to the same procedure as the iodine‐modified catalysts (MgO‐CH3OH) are included in the manuscript. The following order of activity was observed: MgO‐I2 > MgO‐MgI2 > MgO‐HI >> MgO‐ KI > MgO = MgO‐CH3OH. The second part of the study consisted of characterization studies aimed at determining the strengths of Brønsted basic and acidic sites, total concentrations of acidic and basic sites, topography of the surfaces, elemental composition, differences in the chemical environment of the surface ions, as well as relative amounts of different bands in an attenuated total reflectance–Fourier transform infrared spectrum, which correspond to the bonds present on the surface of the samples. Some parameters were independent of the iodine precursor used (eg, the breadth of the Mg 2p signal), whereas others strongly varied from one sample to another (eg, the range of strengths of the Brønsted acidic and basic sites). The ratio of 2 carbonate bands of the attenuated total reflectance–Fourier transform infrared spectra of the modified catalysts shows a trend which corresponds to the order of activity.