Abstract Fluid loss additives (FLAs) based on 2-acrylamido-2-tert.-butyl sulfonic acid (ATBS) provide fluid loss control by reducing filtercake permeability through polymer adsorption. However, when highly anionic dispersants or retarders are present in the slurry, adsorption of the ATBS polymer is hindered resulting in high fluid loss. To overcome this problem, ATBS - N,N-dimethyl acrylamide (NNDMA) copolymers were modified with phosphate groups to enhance their affinity to the surface of cement and make them more robust in the presence of other anionic additives. Fluid loss tests revealed that modification of the ATBS-NNDMA copolymer with phosphate groups greatly improves its fluid loss performance in both fresh and sea water cement slurries. Additionally, it extends its temperature stability up to 150 °C (300 °F), as was observed in stirred fluid loss tests. Furthermore, the excellent fluid loss performance of the phosphate-modified FLA remained unaffected in the presence of AFS dispersant or ATBS-co-acrylic acid retarder, while the effectiveness of the conventional, non-phosphated FLA was severely impeded by these additives. Adsorption measurements illustrated that the superior fluid loss performance can be attributed to stronger adsorption of the phosphated FLA on cement. Adsorbed layer thickness measurements elucidated that the phosphated FLA adsorbs in a "train" like conformation on cement as compared to the conventional ATBS-NNDMA copolymer which attains a "loop"-type conformation.