In a number of heat exchangers there takes place falling film flow on bate tubes installed horizontally in various spatial configurations. The liquid film flow can form various now structures between the tubes: droplet column-droplet column and sheet types. They have been shown in the so-called flow map in Fig. 1. The tests have proved that column-droplet and droplet structures are most common. The formation of droplets and columns is caused by the wave character of the flow and its instability. The method of analytical determination of the liquid-gas interface area for various flow structures has been proposed. The droplet flow structure (Fig. 3) occurs till the dimensionless product ReKa(-1/4) defined by inequality (1) is reached. This structure does not actually ensure complete wetting of the tube surface, so it should be avoided in heat exchangers. The column-droplet and column structures are most often formed during liquid flow down the tubes (Fig. 4) and their formation is limited by inequality (2). For a single segment of in-line tube bundle (Fig. 2) composed of a tube and its neighbourhood the area of liquid-gas interface in relation to the tube length unit can be determined as a sum of the surface of liquid columns and film flow on the tube from formula (3). To calculate such an area it is necessary to know the column spacing which can be determined from formula (6) or for tubes of small diameter, i.e. those that fail to meet condition (7), from formulae (8)-(10) and mean liquid column diameter from formulae (4) and (5). Since the column flow is not a stable form of now, the interfacial waving can lead to the liquid break-up into droplets (Fig. 5). For this to happen inequality (15) is indispensable, the critical length of interfacial wave on the liquid column being determined from simple Rayleigh equation (11). The droplets diameters and their number can be calculated from formulae (12) and (16), the interface area for one segment and column-droplet flow can be determined from formula (19). At the value of product ReKa(-1/4) defined by inequality (20) there begins sheet flow which can be called column-droplet flow. Only at inequality (21) the whole space between the tubes is filled with the liquid forming the so-called sheet now. The sheet flow is an extremely unstable structure of flow and that is why in practice it can occur only at very small distance between the tubes. The interface area referred to tube length unit can be computed for this case from simplified formula (22) derived only from geometrical dependences. The effect of tube spacing was also analysed and in particular the effect of staggered tube spacing on the interface. By dividing the space taken by a staggered tube bundle (Fig 6) into segments and development the liquid space between the tubes, an original method of calculating the interface area to tube area ratio by means of derived formulae (27), (29) and (30)-(32) has been proposed. Diagrams 7a, b illustrate the examples of the results of calculating the interface area to the tube area ratio depending on the ratio of the longitudinal tube spacing to tubes diameters and Reynolds number of the flowing water in in-line tube bundle (Fig. 7a) and staggered tube bundle (Fig. 7b). As it was proved in the paper, the liquid-gas interface area strongly depends on the now structure and tube configuration. The knowledge of the size of this area is very important in modelling the processes that take place in many types of heat exchangers.