(57) Blasting explosive compositions, generally in the form of slurry explosive compositions, which contain ammonium nitrate as the sole or principal inorganic oxidising salt and a methylamine nitrate mixture as the sole or principal sensitizer, are prepared by reacting in the liquid phase ammonium nitrate and formaldehyde to form an oxidizer/sensitizer blend which can then be mixed with other appropriate constituents to form the required blasting explosive composition, the reaction of the ammonium nitrate and formaldehyde being carried out in the presence of urea in order to reduce the amount of free formic acid which is unexpectedly produced in the reaction, since the presence of formic acid in the reaction mixture is undesirable in that it affects the production of a stable gel therefrom and if converted into a formate reduces the strength of the explosive composition.