Process for the joint production of sodium carbonate and sodium bicarbonate crystals, according to which: a solid powder derived from sodium sesquicarbonate, having a mean particle diameter comprised between 0.1 and 10 mm is dissolved in water; the resulting water solution is introduced into a crystallizer, wherein a first water suspension comprising sodium carbonate crystals is produced; the first water suspension is subjected to a separation, in order to produce crystals comprising sodium carbonate on the one hand, which are valorized, and a mother liquor on the other hand; and a part of the mother liquor is taken out of the crystallizer and put into contact in, a gas liquid contactor, with a gas comprising carbon dioxide, in order to produce a second water suspension comprising sodium bicarbonate crystals, which are separated and valorized. A reagent powder comprising sodium bicarbonate crystals made by such process.