Context : The study of physical and chemical properties of massive protostars is critical to better understand the evolutionary sequence which leads to the formation of high-mass stars .

Aims : IRASÂ 18151 - 1208 is a nearby massive region ( d = 3 Â kpc , L \sim 2 \times 10 ^ { 4 } Â L _ { \odot } ) which splits into three cores : MM1 , MM2 and MM3 ( separated by 1′–2′ ) .
We aim at ( 1 ) studying the physical and chemical properties of the individual MM1 , MM2 and MM3 cores ; ( 2 ) deriving their evolutionary stages ; ( 3 ) using these results to improve our view of the evolutionary sequence of massive cores .

Methods : The region was observed in the CS , C ^ { 34 } S , H _ { 2 } CO , HCO ^ { + } , H ^ { 13 } CO ^ { + } , and N _ { 2 } H ^ { + } lines at mm wavelengths with the IRAM 30m and Mopra telescopes .
We use 1D and 2D modeling of the dust continuum to derive the density and temperature distributions , which are then used in the RATRAN code to model the lines and constrain the abundances of the observed species .

Results : All the lines were detected in MM1 and MM2 .
MM3 shows weaker emission , or even is undetected in HCO ^ { + } and all isotopic species .
MM2 is driving a newly discovered CO outflow and hosts a mid-IR-quiet massive protostar .
The abundance of CS is significantly larger in MM1 than in MM2 , but smaller than in a reference massive protostar such as AFGLÂ 2591 .
In contrast the N _ { 2 } H ^ { + } abundance decreases from MM2 to MM1 , and is larger than in AFGLÂ 2591 .

Conclusions : Both MM1 and MM2 host an early phase massive protostar , but MM2 ( and mid-IR-quiet sources in general ) is younger and more dominated by the host protostar than MM1 ( mid-IR-bright ) .
The MM3 core is probably in a pre-stellar phase .
We find that the N _ { 2 } H ^ { + } /C ^ { 34 } S ratio varies systematically with age in the massive protostars for which the data are available .
It can be used to identify young massive protostars .