The effects of an anionic surfactant, sodium dodecyl sulphate (SDS), on the optical transmittance, thermal, and theological properties of methylcellulose (MC) aqueous solutions have been studied. The particular interest of this work was focused on the effects of SDS on the sol-gel transition of MC. Basically, two effects of SDS have been identified, which are the salt-out and salt-in effects at low (<= 6 mM) and high (> 6 mM) concentrations of SDS, respectively. The salt-out effect of SDS is to bring the gelation of MC to lower temperatures, whereas the salt-in effect of SDS is to make the gelation of MC to occur at higher temperatures. In addition, SDS is also able to alter the pattern of gelation. Especially, when the concentration of SDS is greater than 8 mM, SDS not only delays the overall gelation of MC but also changes the pattern of MC gelation from a single mode to a bimodal one. (c) 2005 Elsevier Ltd. All rights reserved.