Many experimental observations exist in the literature illustrating the importance of spread monolayer rheology (mobility etc.) and its influence on the Langmuir-Blodgett deposition process, but systematic investigations of this relationship have received little attention. This paper focuses on the measurement of the in-plane shear viscosity of long-chain carboxylic acid monolayers under a range of subphase conditions. Correlations are made with monolayer transfer behaviour as characterized, for example, by maximum drainage speeds. Also the effect of introducing C=C bonds into different parts of the hydrocarbon chains has also been investigated with an emphasis on ω-tricosenoic acid, CH2=CH(CH2)20COOH. Measurements have been made using a variable-width canal viscometer between two compartments in a Lanfmuir trough. Each compartment could have its surface pressure independently controlled so the effect of the pressure difference along the canal could be studied. Measurements were also made using an oscillating surface viscometer and the results are compared with those from the canal. We discuss results for monolayer viscosity as a function of chain length and time since spreading. These have been correlated with the maximum dipping speed observed in these materials. Results are interpreted in terms of there being an upper limit of viscosity for deposition: too high a viscosity being symptomatic of a brittle monolayer which is merely broken on insertion or withdrawal of the substrate.