Iron—comprising heterogeneous catalyst and a process for producing it, which comprises the following steps:I. thermal decomposition of gaseous iron pentacarbonyl to give carbonyl iron powder having spherical primary particles,II. treatment of carbonyl iron powder obtained in step I with hydrogen, resulting in the metallic spherical primary particles at least partially agglomerating,III. contacting of the agglomerates (=secondary particles) with iron pentacarbonyl,IV. thermal decomposition of the iron pentacarbonyl applied in step III to give at least predominantly pore- and void-free secondary particles.Process for preparing olefins by reaction of carbon monoxide with hydrogen in the presence of a catalyst, with the abovementioned iron-comprising heterogeneous catalyst being used as catalyst.