A continuous or semi-continuous process for the preparation of ethylene glycol from a carbohydrate source including: reacting, in a reactor, at a temperature in the range from equal to or more than 170°C to equal to or less than 270°C, at least a portion of a carbohydrate source in the presence of hydrogen, a solvent, and a catalyst system, to yield ethylene glycol; and, maintaining the ethylene glycol selectivity within a range of equal to or more than 35 % to equal to or less than 85 % by continuously or periodically adding additional heterogeneous catalyst to the reactor, wherein the ethylene glycol selectivity is continuously or periodically determined and compared with a one or more pre-set thresholds and wherein additional heterogeneous catalyst is provided to the reactor if such a threshold is reached or exceeded, and wherein the catalyst system is a catalyst system including: a) a homogeneous catalyst, that is residing in the reactor, which homogeneous catalyst contains tungsten; b) a first heterogeneous catalyst, that is residing in the reactor, which first heterogeneous catalyst contains one or more transition metals from groups 8, 9 and 10 of the Periodic Table of the Elements supported on a carrier, and which first heterogeneous catalyst further contains an amount of tungsten; and c) a second heterogeneous catalyst, that is continuously or periodically provided to the reactor, which second heterogeneous catalyst contains one or more transition metals from groups 8, 9 and 10 of the Periodic Table of the Elements supported on a carrier, and which second heterogeneous catalyst contains no tungsten; or an amount of tungsten in a molar ratio of moles tungsten to moles transition metal that is less than the molar ratio of moles tungsten to moles transition metal of the first heterogeneous catalyst; and wherein continuously or periodically the second heterogeneous catalyst is added to the reactor and wherein preferably the selectivity towards ethylene glycol is maintained by continuously or periodically adding the second heterogeneous catalyst to the reactor. reacting, in a reactor, at a temperature in the range from equal to or more than 170°C to equal to or less than 270°C, at least a portion of a carbohydrate source in the presence of hydrogen, a solvent, and a catalyst system, to yield ethylene glycol; and, maintaining the ethylene glycol selectivity within a range of equal to or more than 35 % to equal to or less than 85 % by continuously or periodically adding additional heterogeneous catalyst to the reactor, wherein the ethylene glycol selectivity is continuously or periodically determined and compared with a one or more pre-set thresholds and wherein additional heterogeneous catalyst is provided to the reactor if such a threshold is reached or exceeded, and wherein the catalyst system is a catalyst system including: a) a homogeneous catalyst, that is residing in the reactor, which homogeneous catalyst contains tungsten; b) a first heterogeneous catalyst, that is residing in the reactor, which first heterogeneous catalyst contains one or more transition metals from groups 8, 9 and 10 of the Periodic Table of the Elements supported on a carrier, and which first heterogeneous catalyst further contains an amount of tungsten; and c) a second heterogeneous catalyst, that is continuously or periodically provided to the reactor, which second heterogeneous catalyst contains one or more transition metals from groups 8, 9 and 10 of the Periodic Table of the Elements supported on a carrier, and which second heterogeneous catalyst contains no tungsten; or an amount of tungsten in a molar ratio of moles tungsten to moles transition metal that is less than the molar ratio of moles tungsten to moles transition metal of the first heterogeneous catalyst; and a) a homogeneous catalyst, that is residing in the reactor, which homogeneous catalyst contains tungsten; b) a first heterogeneous catalyst, that is residing in the reactor, which first heterogeneous catalyst contains one or more transition metals from groups 8, 9 and 10 of the Periodic Table of the Elements supported on a carrier, and which first heterogeneous catalyst further contains an amount of tungsten; and c) a second heterogeneous catalyst, that is continuously or periodically provided to the reactor, which second heterogeneous catalyst contains one or more transition metals from groups 8, 9 and 10 of the Periodic Table of the Elements supported on a carrier, and which second heterogeneous catalyst contains no tungsten; or an amount of tungsten in a molar ratio of moles tungsten to moles transition metal that is less than the molar ratio of moles tungsten to moles transition metal of the first heterogeneous catalyst; and wherein continuously or periodically the second heterogeneous catalyst is added to the reactor and wherein preferably the selectivity towards ethylene glycol is maintained by continuously or periodically adding the second heterogeneous catalyst to the reactor.