Experimental and theoretical study of the spin coating deposition of thin and ultrathin films from dilute solutions of four conjugated polymers, including poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), regioregular poly(3-hexylthiophene), poly(9,9-dioctylfluorenyl-2,7-yleneethynylene), and poly(2,2′-(3,3′-dioctyl-2,2′-bithienylene)-6,6′-bis(4-phenylquinoline)), is reported. Dilute solutions (0.3–2.0 wt.%) of the four conjugated polymers in chloroform were found to be Newtonian fluids with viscosities of 0.7–27.9 cp. The measured film thickness (h f ) of the conjugated polymers was found to be well correlated to the initial solution concentration (x 1,0 ) and the spin speed (ω) by the simple expression, h f =k x 1,0 ω −β . The exponent β is 0.5 for MEH-PPV but is reduced to 0.4 for the other three conjugated polymers. The difference in the β values can be explained by the effect of the accelerative period on the spin coating of less viscous dilute polymer solutions as verified by numerical simulation. A modified Meyerhofer's model was also found to well correlate the film thickness with the fundamental physical properties of the polymers and solvent. These experimental and theoretical results provide a basis for understanding and optimizing the preparation of thin and ultrathin films of conjugated polymers by spin coating.