Apparatus (1) for additively manufacturing three-dimensional objects (2) by means of successive layerwise selective irradiation and consolidation of layers of a build material (3) which can be consolidated by means of at least one energy beam (4), which apparatus (1) comprises an irradiation device (5) with at least one beam guiding element (7) on which the energy beam (4) is partially reflected, wherein a first beam part (10) extends between the beam guiding element (7) and a build plane (18) of the apparatus (1) and a second beam part (11) is transmitted through or scattered at the beam guiding element (7), wherein a determination device (12) is provided that is adapted to determine at least one parameter of the second beam part (11).