The effects of 20 ppm 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP), amino trimethylene phosphonic acid (ATMP), polyamino poly(ether methylene phosphonic acid) (PAPEMP), diethylene triamine penta (methylene phosphonic acid) (DTPMPA), and bis(hexamethylene triamine penta (methylene phosphonic acid)) (BHMTPMPA) on the room-temperature crystallization of calcium sulfate dihydrate (gypsum) were investigated by in situ UV-vis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectrometry (ICP-OES), and scanning electron microscopy (SEM) techniques. A comparison between additive- containing and additive-free experiments showed that BHMTPMPA was the most efficient antiscalant by completely inhibiting crystallization. Due to the chain length of the BHMTPMPA molecule, the crystallization kinetics decreased to a larger extent than DTPMPA. The increase in pH of the solution from similar to 4 to similar to 7 positively enhanced the efficiency of the phosphonates in inhibiting crystallization. Our results revealed that partially deprotonated phosphonate additives were strongly associated with gypsum crystals and/or potentially taken up into the crystal matrix, resulting in a sudden and sharp increase in turbidity plots. Furthermore, phosphonate additives altered the thin, twinned gypsum crystals into thick needles.