In order to obtain ,"eliable find reproducible ,"esults fr01Il dissolution test" it is often neL"essmy to det,,,"ate tbe dissolutioll ?IIedium. 111 tbe Cl/./"rem work, four diff,,"ellt ?IIetbods of degassing tbe medium 7V,,"e compared by mel/r/wing tbe COI/centrotion of oxygen in tbe ?IIedimll before mllil/fter tbe del/emtiol1 p,wed"re. The oxygen concmtratioll was deW"mined "nd,," deftned conditions by using fin o;rygen senso," iiI/sed on a Clm'k electrode" The metbod des{,"ibed ill the USP 23 (beating to 4 5"C pIllS fillnltiol1) I/cbieved an 85% ±ll % ,"edllction in tbe oxygen concentrotion" /-Ieating to 100"C m7t1ted in a ,-eduction of only 50% ±3%, while filtl"lltion ,"edllced tbe oxyg'" cOl1tmt by 65% ±4%. A ?IIetbod developed il1 our laboratm) combinil1g ultrasound and application ofa vacuum achieved fI 95% ±2% ,.ed",1ion. Aft,,• degassing, tbe medill/II must 1HllIdly be u"amiCl-red to tbe dissoltttion vessels before tbe test is mn. Tbe ,"esults indiatt-ed that even careft" filling remlted in a 37% iucrease in the oxygen content, while typical filling proced","es ,"emlted ill an almost tbree-fold increase. It was furth,,' shmv11 tbat tbe medi-Il1Il gmdually becmlles ,"e-saturated dl/ring tbe dissolntiou test itself, at amte that is dependem on tbe apparatlls (basket vet"-S/lS paddle) and the rotational speed. 120 minutes into tbe test, tbe o:cygen COl1celm"fltion was alna(1y 60% oftbe satamtion value" Conclusions; Tbe "least"-e,,,ent of o.rygen was de'lllon-SI1"flted to be a ,"eproducible and dil-ect metbod for mearll1"ing tbe efficienry of deaemtion. It was ftt/1ber sbown thflt the efficiency of dea"wion is bigbly depC1ldC1lt on tbe metbod /lsed, and tbllt "e-introduction of ail" into tbe medium occan not only duringfilling of tbe dissolution vessels but also d","ing tbe dissolution test.