An o—cresolphthalein immobilized cellulose membrane as pH sensitive material was described．The o-cresolphthalein was first reacted with formaldehyde to produeea polymer．The polymer was abbreviated to CPF(o-cresolphthalein—formaldehyde)．Then the CPF was physically entrapped in cellulose diacetate membranes．Finally the membranes were hydrolyzed in 0．1 mol／L NaOH solutions for 24 h to reduce the response time．Advantageous features of the pH—sensitive membrane(CPF—HCDA)include(a)easy of fabrication

(b)a large working range from pH 8．0～13．0

(c)rapid response(2～30 s)and(d)long-term stability for at least six months．So it is a promising material for optical pH sensors and pH test paper of repeated use．